Problem with Prop Valve/Metering Block

I am having the same problem with my 71 Duster. I thought I just had air in the lines but I have bleed them and bleed them. If I pump the pedel once they are fine but if I let it sit then I have to hit the brake twice to get a good pedel. It is almost like I new a residual valve for the back. has anyone every had to run a residual valve on their car. I know they are made for cars that have the master munted to the frame and that doesn't apply to us. but I think it is wierd we are all having the same problem. On my car I have a adjustable prop valve and just using the old metering valve from the drums