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.