THe dual piston brakes were known for being very touchy. And if they are old it's possible they are rocking within the stock bore. Like mentioned above. When you let off of the brakes the pads should not really have any gap in between them and the rotor. It should just have a slight drag to them. The prop valve should only dictate where the pressure goes. The metering valve on the other hand could be the culprit. It is designed to feed pressure to the back first and once there is a set amount of pressure it lets the front brakes engage. It's set up so the car does not do a nose dive when hitting the brakes.