the 91-94 master cylinder is for a vehicle with rear drum brakes, which require less pressure to operate. you can put a 95-01 rear diff with disc brakes on the vehicle and run them w/out a new master cylinder and still stop fine. some people mess with the piston rod on the 91-94 master cylinder or upgrade to a 95-01 mastercylinder as they provite more pressure and volume for the rear disc brakes.
but like DeRocha said.. you do not HAVE to upgrade your master cylinder, just be a little creative with the e-brake cable.
and once again.. searching is your friend.