"Did GM use the same combo valves for front disc/rear drum cars and 4 wheel disc cars? Is that why my rear brakes barely do anything? I've heard of changing the plugs to alter the front and rear bias? Also, I've heard that combo valves are for cars with rear drums. Should 4 wheel disc cars just have a prop valve instead of a combo valve? Sorry, I never knew too much about brakes."

All thirdgens have a combination valve (of which proportioning is a part). The valves are different for drum and disc cars. For best braking performance, use the correct valve. Check the 10-Bolt FAQ for more info.

JamesC

Sometimes the rear brakes do not work correctly because the pads require adjusting, this is done with the hand brake.