Some GM's, like my truck, don't use a proportioning valve with the disc/drum setup. Instead, you get a pressure differential switch. What that means is if there is a pressure difference between the front and rear brake systems -they are seperate, sealed systems- a plunger will move, completing the circuit to the big light in the dash that says brake. That means one of the brakings systems has failed and you should start trying to get the truck slowed down. The switch has no function in regards to stopping the vehicle. So, you may not even need a proportioning valve -you surely don't need the pressure differential switch.