Trailer Brake Problem, releasing the pedal, the brake stays on.

If it’s the same trailer all the time I would check the regulator valve connected to the reserve tank under it.

If it does it with different trailers and you’ve already replaced stuff. Check that the foot valve is releasing air between it and the trailer hand brake valve. Sometimes the foot peddle will not exhaust the air from the line that connects it to the trolly valve causing it to allow air through it without actually applying it. Some have a check valve that is supposed to not let air back flow to the foot peddle when only the trolly is applied. If it goes bad it will cause the brake peddle to apply on it’s own and sometimes you can see the peddle move a little on its own when you pull the handle.

 

Hand brake valve may need replacement. The older ones are serviceable and simple cleaning and a o-ring is usually all they need. Not sure about the newer plastic stuff.
You didn’t say how old the truck is but 10 years out of a good quality Bendix foot valve before they get sloppy is good.

I’ve seen a simple blockage of the exhaust hose caused by a dirt dauber plugging the end of it. Those little boogers work fast and it doesn’t have to sit long for them to start shoving crud in every bolt hole on the side of an engine block or other opening they can find.

 

I use nothing more stronger then alchohol in the valves themselves unless they are torn down and the rubber is to be replaced. I use brake cleaner on some nastier crusty stuff but I try not to let it soak into the rubber parts too long. I spray brake cleaner inside the hoses to clean them out after they have been disconnected from the valves first. Then blow them clean and dry with shop air.

Someone else may have another suggestion.