It appeared that the reason this design did not positively extend the piston rod was to allow an emergency breakaway cable to actuate a lever mechanism and compress the piston independently of any coupler movement. This applies the brakes in the event that the trailer separates from the tow vehicle. However, in normal operation, this allows the coupler to fully extend from the actuator while the piston remains partially compressed. This is a very dangerous design limitation because the coupler/actuator assembly can appear to be working properly even though it is not. Dragging trailer brakes can result in catastrophic loss of brakes or a fire.
Since the coupler did not positively pull the piston rod out when the coupler extended, the only force available to push the piston rod out was the return spring inside the master cylinder. If the trailer had drum brakes, there would also have been pressure in the brake fluid from the return springs in the brake drum assembly that would have helped push the piston back out. However, disc brake calipers do not have return springs, and there is only very light pressure from the rubber piston boots in the calipers. But why wasn't the master cylinder return spring fully extending the piston?
After determining there was no external interference affecting the piston rod, I uninstalled the actuator from the trailer, and removed the master cylinder from the actuator. Upon disassembly of the master cylinder, I found the likely cause of the problem. There was extensive corrosion of the inside wall of the aluminum master cylinder where it is exposed to the atmosphere. This caused drag on the piston cup, which prevented the piston from fully extending.
As a result, the master cylinder retained some amount of hydraulic pressure in the brake line even after the coupler fully extended. This caused the brakes to drag and overheat. However, the actuator itself gave the appearance of functioning properly because the coupler fully extended after each braking action, even though the piston rod inside the actuator housing was remaining partially compressed.
My solution to this problem was to replace the actuator with a different design that utilizes a master cylinder made from composite material that could not corrode.
This entry was submitted by Donald Murphy and edited by Rob Spiegel.
Donald Murphy received his BS in mechanical engineering in 1977, and his Professional Engineering registration in 1979. He earned an MBA in 1983, and in 2011 earned a JD degree and became an attorney. When not sailing his boat, he is CEO of a small industrial manufacturing company.
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