[Blue Dragon]

Heya,
I've got a 1964 Pontiac Bonneville Hearse that I have done a four-wheel disc brake conversion on. The original booster and master have been replaced with a dual diaphragm booster and disc/disc rated master as recommended by MP Brakes. A proportioning valve was installed, but I gutted it as instructed in the directions for disc/disc set-up. ScareBird provided what I needed for the rest and everything fits nicely. The main problem that I have is that the brakes feel too spongy and there is excessive brake pedal travel. I have bled all the air out. When I line-lock all four rubber lines going to the calipers, the pedal is rock-hard. I’m figuring the sponginess is from the caliper pistons traveling out to meet the rotors then returning back in after the pedal is released?? Has anyone run into this problem?? I am thinking of adding two two pound rated residual pressure valves to the system, front and rear, but wonder if will cause brake drag.