i found the solution to my locking rear brakes. A while back I replaced the ugly cast iron mc on my car for a fancy 92 ranger mc because, well, it was ugly. I I have an 89 Mustang 4 cylinder booster that is non adjustable. I didn’t check the length of the booster output rod length before installing the new mc. It is 1/8” to long so my brakes were actually partially on all the time. I installed a new 2000 Mustang v6 mc with spacers. No test drive yet but I’m pretty sure the problem is solved. Live and learn and pass along your stupid mistakes as a learning experience for others.