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.
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.
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 ....
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.
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.
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 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.
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!
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?
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