Another potential problem for disc brake systems can occur if the system is improperly assembled using an actuator intended for drum brakes. Drum brake actuators include a check valve in the master cylinder to maintain a low amount of pressure in the brake line even when not braking. This improves the responsiveness of drum brakes, but isn't enough pressure to cause drum brakes to drag. However, for disc brakes, this residual line pressure will cause the brakes to drag and overheat. So it is important to ensure that the master cylinder does not have a check valve when installed with disc brakes.
Once I understood all this, I could begin to troubleshoot the brake system. The combined boat and trailer weighed around 6,500 pounds, and my tow vehicle is a full-size truck with plenty of power and a tow rating well in excess of the trailer weight. Although towing the trailer makes a very noticeable load on the truck, the additional drag from the trailer brakes caused by low pressure in the brake lines is not noticeable when driving. Therefore, low pressure in the brake lines could be the cause of smoking brakes.
I had successfully towed the trailer a number of times prior to this problem, so I concluded there was not a fundamental flaw with the design or initial installation of the brake system. Nonetheless, I inspected the actuator and noted that it had a label stating it did not contain a check valve and that it was for disc brake systems.
Since the wheels were all smoking at the same time, I also concluded the problem was most likely with a common part of the hydraulic system rather than a common problem at all four wheels. I inspected the brake lines, and they were all clear -- no kinks or sharp bends that might retain pressure. I also checked the individual brake calipers just to be sure there was nothing causing interference. Therefore, the likely cause of the problem was at the actuator assembly as it was the only remaining component in the system that could affect all the brakes.
The actuator on the trailer contained a bypass solenoid, which should relieve any residual brake pressure while in reverse. However, its likely failure mode would be either to not activate when it should (allowing the brakes to be applied when backing), or to have some type of blockage preventing the brakes from applying in the forward direction. Neither of these scenarios fit the problem of low-level brake pressure in the forward direction.
I conducted a simple test of the actuator to ensure it was properly applying and releasing the brakes. I jacked up one trailer wheel, and then simulated the braking action of the tow vehicle by manually compressing the coupler into actuator while rotating the wheel (with a helper). The brake applied and stopped the wheel from rotating, just as the brake should. However, when I released the coupler and re-extended it from the actuator, the wheel remained locked and would not rotate! Ah ha! This confirmed that somehow the actuator was keeping brake pressure in the system. But how? I had released and fully re-extended the coupler from the actuator. There was no check valve in the master cylinder. There were no kinks in the brake lines.
I removed a small cover from the actuator housing and looked inside. It appeared at first that the piston rod for the master cylinder was in a fully extended position, which should have relieved the hydraulic pressure. However, I saw that the coupler only pushes against the piston rod for the compression force. The design does not positively pull the piston rod back out when the coupler re-extends. On closer inspection, I observed that the piston rod was not quite fully extended. I reached inside the actuator housing and managed to fully extend the piston rod. I then rechecked the trailer wheel and saw that it rotated freely. This showed that the master cylinder was keeping pressure in the brake system because the piston did not fully extend upon release of the coupler.
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.
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!
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.
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.
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.
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.
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.
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 
