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.
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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