Testing a Proportioning Valve?

Mine are doing the same thing, and the rear locks up way too early. I suspect that whoever upgraded my 64 dart to duel master with disks in the front, did something wrong. I am considering adding a proportioning valve for the rears. Right now I don't have to step on the brakes very hard before one of the rears locks up. If I ride the brakes for a while it doesn't do it as bad. I have inspected the rear brakes and they are fine, no leakage or glazing. So I am following this