I'm using the PATC O/D kit for the trans and it uses line pressure to engage or disengage the o/d and lockup, and a vacuum switch to read throttle position.
I figured you were likely using a PATC kit. I would absolutely wire in a relay, controlled by the brake switch to disengage lock up at any time the brakes are hit. That’s a safety issue and even if it isn’t your problem, needs to exist.