My trailer's brakes locked in forward when my surge system went bad. Yeah we bled the lines at the brakes just to get home.
I did some tests to see if I could figure what was wrong. The solenoid worked. When tested, I could definitely hear its click and feel its throw.
I just had everything replaced. After that, I conducted an autopsy on the failed parts. What I suspect now is that, the solenoid has a pinhole aperture that must be open, and that the right size of contaminant clogged it and disabled the solenoids function.
The system works kind of like this. It is an auxiliary brake pedal. When you brake your tow vehicle, the momentum of the boat and trailer pushes into the tongue, causing the ram of the surge-brake cylinder to depress into the the surge-brake cylinder. It is like the boat and trailer are stepping onto an auxiliary brake pedal, which applies braking to the trailer brakes.
When you back up the trailer, you again cause the ram of the surge-brake cylinder to depress into the the surge-brake cylinder, which in the absence of a counter-action is going to cause the application of braking to the trailer brakes. The counter-action is the the switching ON of the reverse lights. If the reverse lights are switched ON, the solenoid is likewise energized, and it opens a bypass loop from the output of the surge-brake cylinder to a reservoir of brake fluid for it, which is behind it (in the brake loop ... on mine, the reservoir is actually immediately on top of the surge-brake cylinder ... in the same housing).
When I did my autopsy, I found that the aperture in the solenoid for the bypass loop is teeny ... again ... the size of a small pinhole. I imagine it was at risk of being stopped easily by rather small contaminant particle(s).
The reservoir Tees into the cylinder by an (a final) aperture in the middle of the axial length thereof. When the plunger commences plunging, it first just pushes brake fluid into the reservoir. Once the plunger closes the aperture to the reservoir, pressure is applied to the brakes. When the trailer is pulled forward, that sliding axial pin you see in your trailer (and which you lockout with nuts or coins and the like) is actually just locking out a shock absorber. It appears that the force which returns the plunger to the extended position are a pair of springs ... the heavy duty work probably being done by the red one, given its size relative the little wire spring.
When my system was contaminated, I imagine the following happened. It agrees with the sequence of things.
It was getting difficult to back up my trailer. My brakes were constantly braking while backing up. I believe the bypass aperture was clogged. So, several times I just gunned my tow vehicle in reverse, causing the tires to sort of bounce or otherwise break free a little. What I was doing was causing the plunger to travel past the last aperture to the reservoir. Once I did that, and with the bypass aperture being clogged and allowing no bypass flow to the reservoir, I had my plunger stuck by the incompressibilty of the brake fluid. The springs could not supply sufficient 'break free' force to push the plunger back behind the last aperture to the reservoir against the 'hold-fast' tightness of the plunger being jammed past the normal travel it should travel in the cylinder.
Disclaimer. This is the Internet ... where everyone is smarter than everyone else. Don't come back and tell me all it took was a rap with a hammer (which might not be a bad idea).