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?" :)
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.
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.
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.
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.
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