Rear brakes lock up... front disc

well you need a adjustable proportioning valve, my duster was doing the same thing so I bought one, didn't install it as I found out my axle was leaking fluid the shoes were coated with gear lube and that was causing my issues, if your not leaking oil onto the shoes than you might need the prop valve.

http://www.summitracing.com/parts/DCC-5249088/?rtype=10