just fixed this problem on my car, all 4 were slowly locking up as i drove around town
there is a pushrod in the brake booster that may need adjusting
there needs to be clearance between the pushrod and the master cylinder. So adjust it 1/4-1/2 turn at a time until its fixed.
basically unbolt the master from the booster, move it out of the way being careful not to kink the lines, have someone push the brakes all the way down slowly, then fish a pair of pliers into the hole to grab the knurled portion of the shaft and use a 7mm wrench to turn the nut inward.
if you go too far youll have a ton of pedal travel, just open it back up and adjust a bit less the other way.