If two different trucks exhibit the same problem when towing the same trailer, I would first suspect the trailer as being at fault. If you could tow the trailer with a third vehicle, one that has a known properly working controller, and still have the same problems, then the fault can be directly attributed to the trailer. Normally an abrupt engagement or release is caused by an errant controller. If the brakes go on hard, generally the magnets are getting too much voltage, too fast, and this is adjusted at the controller. The trailer may be wired so that it is getting additional voltage from another source, through a short or an improperly grounded wire.

If it is the trailer's problem, you can have one of two problems or a combination of both; mechanical or electrical. An electrical malady from day one could indicate the magnets are improperly activating, again usually from too much voltage, but it could also be a wrong part initially installed when new. Depending on the weight rating of the axle and their age of manufacture, the magnets' sizing can vary accordingly. New magnets can be installed if they are found to be defective. The brake wiring is quite simple, a heavy wire going from a single source at the plug, running aft along the frame and then split off to each brake assembly. The brake magnets have two wires, one the hot side, the other a ground. It is a very simple system to trouble shoot.

Another issue is mechanical. The brake shoes contact the drums by a linkage system. The magnets grab the flat surface of the drum, which then moves the shoes against the inside circular part of the drum. The resulting friction applies the braking action. The drum's flat surface should be free of rust and smooth, with no grease or oils. The drums' curved surfsaces should also be smooth, with no grooves, cracks or chips. They too should be grease free as well as the brake shoes.

If the linkage is frozen, or wrongly installed, or damaged or has pieces missing, the braking effort will be errant. If the shoes are the wrong size, or have the wrong material, are worn thin, or are rusting to the drums, they can cause improper braking action. Adjusting them will not usually help with these problems.

If these units have not been maintained or changed for 15 years, I would consider purchasing completely new brake units. They cost ~$100+ an axle depending upon the axle ratings, and contain pre assembled all new parts, for a completely new braking system. They are less labour intensive to install than individual replacement parts, and you are assured of having the correct, working, compatible, new assemblies.