Before you changed the master, was the fluid level low? Check for leaks in the system? And when does the problem tend to occur? at random or after you've been driving for a while?
1996 Jeep Cherokee- Project-- Rusty's 8in long arm, SYE kit, Teraflex driveshaft, Cragar soft 8's 15x10 with 33x12.5 Firestone Destination MT's, Eaton Tru Trac in the front