Trailer breaks staying engaged

Sounds like your lines may be hooked up backwards.

 

"When you apply the brakes" are you using the foot pedal? Spike? Are your red and yellow buttons pushed in or popped out?

If his red and blue lines are backwards the trailer would still not move, that'd mean red line is not filling the spring brake side of it.

I'm thinking more a stuck valve. I had it in my foot pedal once, gunk on the floor wouldn't let it fully come back up, lift pedal it was fine. A good cleaning and it's fixed. Perhaps there's a similar valve in the spike side of it just isn't fully releasing for some reason.

Op do you have a brake application gauge?

 

I know another guys that does that on occasion, it’s done bendix brake sensor,
Trailer won’t release for 20-30 seconds, brake light stays on, then it’ll release

 

I’d start with the tractor protection valve. It should be right where you lines connect to the tractor.