You can try opening the bleed valves on the calipers and pumping the actuator. I don't know what kind of brakes FW used on those trailers, but I have heard they are the vented Tie Down ones, if so they have a website with installation instructions and if you have a Tie Down actuator, they also have instructions on how to manually pump the brakes, and bleed them. There is no manual release per se for brakes. You have either a problem (blockage) in the line that lets the brake fluid return to the master cylinder, or you have one or more disc brake caliper pistons sticking. The back up solenoid should not have anything to do with this problem.
I have heard that disc brakes are so much better than drums, but I also have heard over the years of a lot of problems with them not releasing. The issue with disc brakes in a surge system is that disc brakes depend on the calipers not sticking and the fluid being able to flow freely back to the master cylinder to release the brakes. Drum brakes on the other hand have strong return springs to release the brakes. As long as you can keep water from corroding the wheel cylinder, they will release. Drum brakes also are self energizing, which means that they provide better braking torque at the same actuator pressure.
http://www.tiedown.com/pdf/c752.pdfhttp://www.tiedown.com/pdf/c553.pdfhttp://www.tiedown.com/pdf/c100.pdf