There's 2 ways to lock a brake up.
-Mechanical
-Hydraulic

Finding which 1 of the 2 ways is locking your brake is easy.
Like I posted above...jack it up, pump brakes till it locks, crack the bleeder screw. If fluid comes out and the brake releases, problem is in the hydraulic system.

Changing parts like the booster (which isn't your problem) is a waste of time and $$$, diagnose the problem first.