from your discription of the pedal action[slow to return & showing some resistance] i think the booster has problems. as for the pedal ratio[4.28 : 1] it is in the ball park for power brakes. 7:1 would be a good ratio for manual brakes. not sure on the chevy truck calipers, but if they are close to the vette [4 piston ?] area size wise, this shouldn't be a problem. if the proportional valve also has residual valves in it, that could be a problem for two reasons. one, with your master mounted on the firewall, you don't need 'em[residual valves] as they are for use when the calipers & wheel cylinders are above the master mounting height to prevent fluid drainback from the cylinders & calipers into the master. 2nd, if in fact they[residual valves] are for a disc-disc setup, they would be[should be] a 2lb. valve[s]. are the brakes retaining heat, as in not releasing fully ? this could have glazed the pads & linings. just thowing out things here that may be of help. brakes can SURE be a pain at times......