In operation, when the tow vehicle slows, the inertia of the trailer presses the actuator against the coupler, pushing the piston into the master cylinder. Like stepping on the brake pedal in a car, this applies pressure through the brake lines to the individual brakes at the trailer wheels. The system self-regulates the amount of braking force because the force on the piston is proportional to the rate of slowing of the tow vehicle.
The more the tow vehicle tries to slow, the more force is applied to the piston and through the hydraulic fluid to the trailer brakes. Conversely, when the tow vehicle accelerates, the hitch ball pulls the coupler forward, extending the piston and releasing the brakes. A very simple, cool design.
However, an unintended consequence of surge brake systems is that the actuator also applies the brakes when backing up because the tow vehicle is pushing against the inertia of the trailer. On flat ground with a light trailer, this may not be a problem. However, it can be a big problem when backing up a hill or driveway with a heavy trailer, especially if the trailer is equipped with disc brakes instead of drum brakes. Therefore, the brake system requires a method to deactivate the brakes when in reverse.
Some actuators allow the driver to deactivate the brakes manually by temporarily inserting a lockout pin into the actuator to keep the actuator/coupler mechanism from compressing. However, the pin will prevent brake actuation both in reverse and in the forward direction. This presents a serious safety hazard because if the driver inadvertently leaves the lockout pin in place while driving forward, the trailer brakes will remain deactivated.
A better approach is to use a system that automatically deactivates the brakes only when backing up. In this respect, drum brakes have an advantage over disc brakes because the design of the brake shoe mechanism in drum brakes leverages much of its braking force from the forward rotation of the drum. As a result, drum brakes are very ineffective in reverse, so backing up is not difficult even if the brakes are applied. In addition, some drum brakes utilize a "free-backing" design, which virtually eliminates all braking action in reverse.
Disc brakes, however, are equally effective in forward and reverse because the braking force is independent of the direction of rotation of the rotor. To prevent braking in reverse, actuators intended for disc brakes often utilize either a blocking or bypass solenoid. The solenoid mounts between the master cylinder and the brake line, and connects to the backup lights of the tow vehicle. Therefore, the solenoid actuates only when the vehicle is in reverse gear, eliminating the potential of the brakes remaining deactivated when driving forward.
As the name suggests, a blocking solenoid blocks the brake line, preventing pressure from the master cylinder from pressurizing the brake line. However, a blocking solenoid can still result in the trailer brakes activating while in reverse because the solenoid will not relieve existing pressure that can already be in the brake line. This will occur if the driver stops while traveling downhill and shifts into reverse to back up the hill. Even though the blocking solenoid will prevent the master cylinder from applying additional pressure to the brake lines, the existing residual pressure in the brake lines can keep the brakes applied.
As an alternative, a properly designed bypass solenoid relieves the pressure from both the master cylinder and the brake lines by opening a bypass port into the brake fluid reservoir. This prevents the master cylinder from applying new pressure to the brake lines, and relieves any existing residual pressure in the brake lines.
Good job troubleshooting and repairing the problem! I never fully understood the operation of surge brakes, but I have a good understanding now.
Every time I end up delving into an odd situation like this and finding a solution, I always wonder to myself "What do normal people do when things like this happen?" :)
I wonder the same thing. Of course, there are other times when I take something apart and can't get it back together that I realize sometimes normal people don't have it so bad.
I can't believe that response never occurred to me ..... And coincidentally, I just completed a Bathroom renovation at home that included re-routing plumbing lines. My lovely wife DID use that very line on me, and I STILL missed the response ....
Good job on supplying the detail of surge brake operation. You would not normally expect to see corrosion inside the master cylinder which isctypically a sealed environment. Good article.
Good article with a clear thorough explanation of the mechanisms. As I was reading, I mentally concluded that the actuator simply needed a stronger, more robust "return" spring. I wouldn't have continued into the internal cylinder and discovered the corrosion! Good work - Very thorough.
Hydraulic brake systems are not maintenance free. Brake fluid, being hydrophilic, will absorb water over time and the corrosion seen on the aluminum piston/bore is the end result. Bleed your system out yearly refilling with a high quality brake fluid from a fresh, unopened container and you will greatly reduced the chances of this type of failure.
I agree that boat trailers are not maintenance-free.
However, I respectfully disagree with your general statement that all brake systems need to be flushed annually. I am not sure what kind of car you drive, but none of the Fords, Chrysler, or GM cars or trucks I have owned specified hydraulic system flushing as a periodic maintenance item. This is not necessary- it is about equivalent to changing your oil every 500 miles- it can't hurt, but it's just not necessary. If it were, manufacturers would certainly recommend it to avoid liability and provide dealer service departments with additional revenue.
Automotive hydraulic systems can (and do) last for decades without being flushed. Usually the only brake system maintenance that is required is replacement of linings and resurfacing or replacement of drums/rotors. When seals or hoses crack or start to leak, then it is time to replace them and flush the system while the hydraulic system is open.
BTW, bleeding and flushing are two different tasks. Bleeding is usually done to remove air, not to remove all the fluid.
I've never seen anybody off-load a boat without backing the trailer all the way into the water. All hydraulic master cylinders have an air vent, otherwise the returning fluid woulld be trapped by a pressure gradient. Water can and will get into a boat trailer brake system. Preventative maintenance is the key to longeveity and reliability in any type of vehicle or equipment. True, it may not be necessary to flush annually on your street driven grocery getter but the same does not hold on a boat trialer. Which, I think, is what we were taking about.
The air vent is closable and used only for bleeding air from the system. If it were left open, the brakes would not work. Hydraulics are necessarily a closed loop system.
Water infiltration to the system happens when the brakes are submerged to a depth that applies sufficient water pressure to the outside of the seals. This pressure along with the mechanical motion of the piston action flexing the seal can allow water into the system. The brake components don't go very deep, but the wheel pistons can get down to 5-6 feet when the brakes are set during launch and recovery. But remember that within just 33ft of the water's surface, standard atmospheric pressure is doubled.
My comment was about the fill cap on the master cylinder. The fill cap on a hydraulic brake master cylinder is vented to the atmoshphere. Water and or moisture laden air can and will get in to the reservior where the fluid will grab the moisture and then proceed to corrode the innternal parts given enough time.
I still agree that boat trailers are not maintenance free.
Some automotive brake systems use a flexible membrane between the fluid and the air vent, so air is not in contact with the fluid all the time. If water were to enter the vent of a system like this, it would not necessarily get into the brake fluid.
Brake fluid is glycol based and quite hydroscopic. Periodic fllushing will clearly show the old fluid as darker (from rust) and the new, fresh fluid is water clear. Old glycol brake fluid with a high moisture contect has a lower boiling point and causes corrosion. Disc brakes can get hot enough to boil brake fluid during heavy use so flushing is specified as part of the normal maintenance cycle. Troubleshooting surge brakes can be extremely frustrating. People don't learn they have a problem with their brakes until they either crash when the trailer brakes don't work, or they have smoke when they don't stop working. Good troubleshooting and interesting article.
I wonder if it would make sense to use DOT5 brake fluid for boat trailers? If I understand correctly, DOT5 brake fluid is silicone based and non-hydroscopic (actually it's hydrophic).
I know that they don't recommend converting normal systems to DOT4, but maybe it would make sense for a boat trailer?
With regards to normal brakes, I have done a few "track days" in my Trans-Am, and once a season would flush the brake fluid. I use ATE Super Blue and ATE Gold for brake fluid. They have a high boiling point and are different colors (one if blue, one is gold) so when flushing the system, you know when all of the old fluid it out.
I've also flushed the fluid on my motorcycles too when upgrading to braided stainless lines.
Well, my 2000 Silverado truck also runs ATE super blue too, since the factory brake lines rusted out (!!!) and I had to install aftermarket stainless lines this year. One of the worst repair jobs I've ever done myself....
To Jim_E, regarding the use of DOT5 brake fluid, it gets complicated. I believe you are correct that DOT5 is silicone based and therefore doesn't absorb water. In addition, it has a higher boiling point than DOT3 or DOT4, which is generally good.
But, the big factor is that silicon based fluids (DOT5) cannot be used for brake systems that are not designed for it, else it will damage the seals. Maybe someone with more specific knowledge about brake systems and fluids can chime in.
Once upon a time I used DOT5 on my motorcycles. There's no issue with seals, since DOT5 is very inert and benign compared to DOT4. Everything must be thoroughly cleaned before changing between petroleum and Si-based fluid.
All brake systems collect moisture through the master cylinder reservoir. Since DOT5 is not hydro/hygro scopic, water will pool in the low points, usually the brake calipers. I would replace the DOT5 annually and overhaul the calipers, which would often have signs of corrosion at the bottom part of the piston and bores.
In their wisdom, the government came up with DOT5.1, which is petroleum-based and incompatible with DOT5 ! Talk about setting things up for human error....
Typically, an automotive master cylinder is actuated by a rod, either from the brake booster or directly from the pedal. The rod is not generally directly attached to the piston in the master cylinder. It is not uncommon for the piston to hang up in the bore after brake release, and for the brakes to drag a little. This is not usually a problem, and the brakes don't usually drag to the extent that they smoke.
It doesn't surprise me that there would be corrosion in a boat trailer brake cylinder. Boat trailers are often submerged in water, and sometimes in salt water! The brake cylinder used in a corrosive environment like this should either be well-sealed or made of materials that don't corrode, like the one that you installed.
It's been my experience that normal people either pay someone else to fix it or just live with the problem for ever. Fear seems to be the biggest problem most have in never trying to repair anything. I can't tear that apart, I will never get it back together, or I won't be able to find the parts, well this stops almost all repairs right there. Being an engineer, we love to dwelve into the guts of the problem, are never satisfied until we have found the root cause, finding joy only when we got that hidden gremlin exposed. Engineers don't fail, don't break things, and never stop trying, we are only adding to our knowledge bank for the next problem.
As a kid, I used to tear everything apart. Now, as an adult, I have a healthy fear of some engineering missions. It is never a good idea to start troubleshooting your only vehicle for a minor problem on a Sunday when you need to go to work on Monday. This is safer to take to a professional.
Well, yeah, but the right solution is a backup car, not using professional services that may not be as careful and meticulous as you are.
Reminds me of a time when four of us engineers carpooled together, back in the 70s. One individual, Tom, was like you--take the car and TV to "professionals" for fixing. He mentioned his frustration--he had taken his car to four garages and it still didn't run right when it was cold, common in Minnesota where we lived.
Another of us, Curly, said he would fix the car, but wanted to see the invoices so he could see what had been done. He found that the first guy had replaced spark plugs and ignition points and condenser. These were common wear-out parts that had to be replaced every 10,000 miles, and were probably replaced without even checking to see if they were worn. The second guy had also replaced the plugs, points, and condenser. As had the third and the fourth.
Curly concluded that he probably didn't need to even look at those items. He poked around for a few minutes and found that the heater tube which carried hot air from the exhaust manifold to the carburetor to prevent carburetor icing had rusted/burned through right where it exited the exhaust manifold. A $3.00 replacement at the auto parts store and fifteen minutes to install it solved the problem.
As we talked about it, we concluded that all four shops had probably diagnosed and repaired the car inside a warm garage, not in the cold environment where the problem actually manifested itself.
So the question, Tim, is "Do you really want to let a 'professional' do the work, and how many trips back to you care to make?"
I understand your point that as an owner you would take better care of your stuff thanothers. Nine times out of ten, I would perform required maintenance on items that I own, but sometimes, it is better to have someone who has more experience and training work on an issue.
I worked part time for a mechanic, for gas money while I was in school. I had always worked on my own vehicles, but that experience removed all the 'fear of fixing' from me. The problem I have now is that I don't want to do that work anymore, but I have yet to find a 'professional' mechanic that does work to my standards.
You are very correct that you must plan out the repair. You need to be prepared to spend the time and money to do it right or it will be pointless for most cases.
Watashi wrote: " worked part time for a mechanic, for gas money while I was in school."
Yeah, me too,
And he wrote: "I had always worked on my own vehicles, but that experience removed all the 'fear of fixing' from me."
Me too, mostly. I don't attempt alignments, internal transmission repair, or the gaseous side of air conditioner repair.
And he continued: "The problem I have now is that I don't want to do that work anymore, but I have yet to find a 'professional' mechanic that does work to my standards."
About once a decade I find a job I don't feel like doing and hire it out. I get tired of taking it back and explaining what was done wrong. That seems to set me straight for another decade,
I have also learned not to take things apart that are going to be needed to do something before you can get it back together. i.e. do the laundry and then take the washing machine apart.
The best system for surge brakes would be a solenoid valve that enabled them whenever the tow vehicles brakes were activated. That would remove the backing problem and the downhill problem as well.
I would have suspected corrosion someplace as a first candidate, since it seems that many designs of disk brakes are not even designed to be water resistant. At least, that was my experience with several of the 1970s Plymouth and Dodge products. The caliper pistons would rust ijnside, and the steel-on-steel sliding surfaces outside would rust, and so the caplipers would engage but not release. ON a boat trailer both the calipers and the master cylinder would automatically be suspects. And why would anybody make a brake system out of materials that everybody knows wil corrode quickly?
Thanks for all the comments. Here is more information that may help understanding some of the challenges I faced.
Air Vents.
All the master cylinders that I inspected have an air vent to the fluid reservoir, usually in the fill cap. This allows the fluid level in the reservoir to fluctuate to accommodate brake pad wear, thermal expansion, etc. However, many (all?) also have a diaphragm under the cap to keep out moisture from the atmosphere, rain, etc. In my case, I don't really know if this is sufficient to keep water out when the actuator is submerged during launching (under a few inches of water for a few minutes). This alone is good reason for me to flush the brakes regularly (which I do).
I don't think water will enter the system from the piston seals at the brake calipers, as those seals are designed to withstand the relatively high pressure of the hydraulic fluid when braking. The static water pressure against the seals from the outside when submerged is quite low, maybe 3 psi (6 ft. / 33 ft. X 14.7 psi). Also, the pistons don't move when under water because the brakes are inactive when launching (see below).
Why the Actuator Gets Submerged.
Contrary to the design of my trailer, the actuators for most boat trailers are not submerged during launching because the coupler remains attached to the hitch when launching. The actuator doesn't get into the water without the back of the tow vehicle being submerged (not a good thing).
My trailer is unusual because it carries a sailboat with a fixed keel. With the keel, the boat sits high on the trailer, so the trailer has to go relatively deep in the water before the boat will float off the trailer. To do this, the trailer has a separate extending tongue and coupler that is used only for launching.
Before launching, I completely unhitch the trailer from the tow vehicle, extend the launching tongue on the trailer, then hitch the tow vehicle to the coupler on the extended tongue. Since the surge brake actuator stays with the trailer frame, it is quite a few feet behind the tow vehicle when launching. Therefore my actuator gets submerged when launching, even though the back of my tow vehicle is out of the water.
Since launching requires use of the launching tongue, the brakes are totally inactive during launching because the coupler at the actuator is disconnected from the hitch. However, this is not a problem since the trailer brakes are not needed at the slow speeds used when launching.
Using A Professional
Why didn't I get a professional to do the repair? In this situation I didn't have much choice. I couldn't take the trailer a repair shop because I couldn't tow the trailer with the brakes dragging. A tow truck wouldn't have helped because the problem was the trailer, not the tow vehicle. Further, the trailer is too long to fit on a typical flatbed tow truck, so it would have required an expensive tractor trailer rig for moving large equipment. There also could be possible bridge clearance issues because the height of the boat and trailer even on the ground is already 12 feet high. And I couldn't take the boat off the trailer because I couldn't get the trailer to water. Catch-22!
On the scary side, one trailer repair shop that I talked to suggested that my best option might be to just tow my trailer to the shop by disabling the brakes. When I asked if he was kidding, he said I would be surprised at the number of boat trailers that do not have functioning brakes. Corrosion, maintenance and repairs can be such a headache (as I have found out), many people just disconnect the brakes. I now give a wide clearance to any boat trailers I see on the road!
When you haul a boat on a trailer, the wheel bearings get warm, possibly even hot. When you immerse the trailer into the water, the steel cools rapidly creating a partial vacuum in the wheel hub which draws water into the hub, diluting the grease. Expensive trailers have buddy caps which are spring loaded bearing grease caps that keep the grease under constant pressure during the colling period. The heat that is generated from rolling and braking also heats the calipers. When the brakes are not applied, there is no, or very slight movement of the pistons within the calipers, so when the fairly large caliper cools, it also tends to create a partial vacuum which, depending on the condition of the dust (dust, not water) seals will permit water to come in contact with the piston. The caliper on a trailer wheel is almost always steel and that small amount of water which is potentially ingested every time the trailer contacts water will create corrosion. I am not confident a "professional" would have found this problem within the budget of the boat/trailer owner. It was a good fix though, glad it wasn't me.
To bob from maine, those are good thoughts. My short answer is that since my boat is a sailboat, the trailer components have about an hour to cool after towing before they touch the water due to the time required to setup the boat (raising the mast, etc.). This is unusually sufficient time to reduce the temperatures to near ambient.
Also, I should have made it more clear that the extensive corrosion I found in the master cylinder occurred where the cylinder was exposed to air - behind the piston cup, not the front side where the hydraulic fluid is contained. The only protection the back of the cylinder had from moisture was a rubber boot covering the piston shaft and the back of the master cylinder. I haven't seen any problems (yet!) with corrosion in any parts of the system that are in constant contact with brake fluid.
Yikes, all 4 wheels were smoking? The two times (different cars) when I've lost my automobile brakes on a long, steep mountain road, only one of the wheels smoked. I know the smell of burning brake fluid very well. Looks like this Sherlock was a particularly tough one to solve.
Remind me not to go down a steep mountain road with you. I have never known anyone who lost their brakes like this, and twice? Do you ride the brake? Whoa!
Remind me not to go down a steep mountain road with you. I have never known anyone who lost their brakes like this, and twice? Do you ride the brake? Whoa!
Warren, the first time I was young, inexperienced on mountain roads, and alone. I didn't know how to use lower gears, even on an automatic (both cars were automatics). In both events, only one of the brake sets went out, which is why I live to tell the tale. The second time I was highly experienced and not alone--fortunately, the guy who was about to be my husband said not a word and let me do my thing--and people had died on that slalom-like section--so we lived. In both cases, there had been no warning signs of any kind, such as brake noise, and the second car was well maintained. I know several people who've had similar experiences, more than once, and Highway 1 has more than one "Deadman's Curve" sections (title of a Beach Boys song way back). Perhaps your mountain roads aren't as steep or as windy as they are in the Coastal Range in California.
Warren, no offense taken; it was clear you don't drive the kind of roads I do. Those "Deadman's Curve" labels are not symbolic or imaginative, so my luck was actually quite good. The big problem I see is not enough warning symptoms before failure occurs, as well as brakes not designed to handle the amount of use required on long descents, no matter how much you use lower gears. We've got deer, too, up here and one took out the entire front end of my first husband's 1965 Mustang, which had a V-8 and a heck of a lot of steel in it. I bet you've got some great mud stories, though--and that's *not* sarcastic.
You're right about the roads in California. I'm from Chicago and my first time driving in California I was shocked by how tight the turns are on the exit ramps.
tekochip, that's a whole 'nother subject that many drivers have complained about here: tight curves on exit ramps, as well as very, very short exit ramps in some places. OTOH, when driving in New England, especially Massachusetts, many years ago, I was amazed at how poor the freeway exit signage was in letting people know what exits to take to get to specific places, (XYZ village name instead of Highway XX North, Highway XX South). It reminded me of some areas of Britain: you had to be a local to understand where you were going and which road took you there. The last time I drove there, in the early 2000s, I did see some improvement.
Warren, Ann and tekochip: Here's some more info about the driving conditions I was experiencing when the trailer brakes started smoking. That day I had only driven a short distance with the trailer, all on level roads. Before entering the freeway, I had driven less than two miles from home through town at a maxim speed of 35 mph. When I entered the freeway, the brakes started smoking within the first 500 yards. I was not using the brakes because I was just getting up to freeway speed.
Regarding riding the brakes, some interesting dynamics occur with surge brakes. In steep downhill situations, controlling the speed of the tow vehicle by using a low gear and not riding the brakes does not avoid the potential for the trailer brakes to overheat. Regardless of how the speed of the tow vehicle is controlled, if it creates a great enough resisting force against the trailer inertia, the surge actuator on the trailer will apply the trailer brakes.
One reason I was so surprised with the overheating was that just the week before this problem occurred I had towed the boat on a 500 mile trip. That trip included mountainous roads and 100 degree air temperatures. Although there were problems with individual brakes on the trailer, there was never any problem of all four wheels smoking. Interestingly, this trip included going down "The Grapevine" grade on Interstate 5, the main north-south highway in central California. This grade is about 6 miles long, has a 6% slope, and includes escape ramps for runaway trucks whose brakes fail. Despite the potential for the trailer brakes to overheat, the brakes appeared to handle it fine, with no smoking.
This post put forth more information than I have ever read in one of these columns and I now know more about trailer brakes than I did before. Well done. However, once you decided that all four wheels were locked, why did you not go immediately to the only thing they all had in common? The master cylinder. It was almost as if you were avoiding the actual problem to find other things you could fix.
Just curious; does anyone know if pendulum (sp?) brakes are still used on smaller trailers?
To Tool_maker: Thanks for the kind words. As far as suspecting the master cylinder early on, I actually did suspect it. But when I checked it, everything looked normal as far as I could tell (with what I knew at the time).
And yes, I was trying to avoid concluding the master cylinder was the problem. I didn't really want to disassemble such a major component without knowing what I was looking for, much less whether I could fix it or replace it. Without the benefit of hindsight, I could not think of what type of defect in the master cylinder could cause the symptoms I was experiencing. And as it turned out, it took a quite a bit of research to find a suitable replacement actuator/master cylinder, and it was a pain to install it on the trailer frame. And it was expensive to boot.
By the way, what is a pendulum brake? I haven't heard of that before. Is it an actual brake mechanism, or a brake controller, or something else?
A pendulum brake was a mechanism that attached a wieghted lever to a mechanical brake. When you applied the tow vehichle brake, the pendulum swung down and moved the mechanical brakes into position to rub shoe to drum. It seems there was a pendulum on each wheel and since I was just a kid, I do not really know if it was effective. They were on a light weight trail used to haul a 14' aluminum jon boat.
I still have a boat, but have not used it in years. I am about to buy a new truck and want trailer braking.....I never thought about what you mentioned ...thanks!
There is a basic flaw with the very concept of surge brakes, which is that there is no way to apply them without braking the tow vehicle. I am aware that they allegedly have a breakaway application device, but that is of no use when you simply need to apply brakes on the trailer, which you need to do non the occasion that the trailer is forced into an oscillation mode, either by the shockwave from a truck passing too close, or a number of other causes. And if your vehicle brakes fail, that surge brake is not available to slow you at all. So the first step is to replace the surgen brake with a system that can work. One "older concept" system uses the tow vehicle's power steering pressure and a control valve piloted by the tow vehicles brake system pressure to activate a hydraulic cylinder on the trailer that drives a secondary master cylinder. That system can have an electrical valve to provide remote operation of the trailer brakes, for emergency and parking use. Other systems have used tow vehicle engine vacuum to drive the trailers master cylinder. Of course, all of them cost more than a surge brake system, but all of them beat driving without any brakes.
I have stopped nmy van, while pulling a travel trailer, using the trailer brakes alone. It is not nearly as good as the four-wheel brakes were, but I did get stopped. That was when a front brake line bust on my 1985 Dodge van. Contrary to the claims, when you lose front brakes, you have no back brakes either. At least, not on a Dodge from that model year.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 5
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
For industrial control applications, or even a simple assembly line, that machine can go almost 24/7 without a break. But what happens when the task is a little more complex? That’s where the “smart” machine would come in. The smart machine is one that has some simple (or complex in some cases) processing capability to be able to adapt to changing conditions. Such machines are suited for a host of applications, including automotive, aerospace, defense, medical, computers and electronics, telecommunications, consumer goods, and so on. This radio show will show what’s possible with smart machines, and what tradeoffs need to be made to implement such a solution.
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
7 
