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.
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!
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.
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?
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?
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....
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.
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.
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.
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....
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