I have a53 Chevy Bel Air that I put a Turbo on. Had to move the Master Cylinder from the firewall back under the floorboard. Had good brakes when MC was on the firewall. The front end is a Jaguar with dice brakes and the rear are drum brakes. The MC has a 1 in. bore Corvette. It has a proportioning valve and 10-pound residual valve on the rear and a 2-pound RPV on the front. It has a 7 in. vacuum booster, vacuum pump and a vacuum canister reservoir. It is pulling 22 in. of vacuum at the brake booster. The rod between the booster and the MC is apparently 3/16 of in. of play. The brakes have been bled and brakes adjusted several times.

Now the problem; hard brake pedal. I replaced MC, proportioning valve and brake booster to no avail. Had it to a brake shop and $301 later it's no better. Does anyone have any suggestions?.

What is your pedal stroke ratio? It should be about 6:1 for power brakes, 5:1 is about the minimum limit. Start there.

Run a smaller diameter MC, maybe 7/8" instead of 1". The pedal stroke will increase a little, but the hydraulic advantage between the MC and the wheel cylinders and calipers will improve. Chances are the under floor booster isn't a large enough diameter to exert enough force on the fluid.
Jerry