I have a 1983 z28 I replaced all four calipers rear brake line front brake hoses and master. I bleed all the air out brake pedal still sponge and sinks to the floor. After putting in a booster canister being fed from he carb then from canister to booster the brake pedal is rock hard. When you start the car and step on the brake the brake warning light comes on then goes to the floor but if you double tap the brake pedal the brake warning light turns off and the brakes works the way it should then step on them once the light comes back on and goes to the floor any ideas

 

I posted this in another thread where someone was having possibly a similar issue. If the rear hoses are the stock piece throw them out and purchase a new one-- anything is better than that >23 year old hose you may currently have. Make sure all hoses have copper crush washers on them and they are TIGHT. Make sure each caliper has the bleeder screw at the TOP of the caliper and NOT the bottom. If the lines were ever run dry, disconnected from the master cylinder or the master cylinder was new then it NEEDS bench bled and then all the lines rebled and you need to ensure it never goes dry again while bleeding. Use a clear hose when bleeding and use one of the cheap one person bleeder kits with the vacuum pump for ~$30, I think you can even rent them. After that start the car and pump the brakes several times and then bleed each line several times while smacking the caliper with a mallet. Tighten all lines and connections and bleeder screws and press the brakes hard several times and inspect for fluid at a connection or on a line. It is possible that a hard line has a split or crack in it. If you find no fluid after all of that than it is almost certain the problem is mechanical be it a bad master cylinder, stuck pistons, glazed rotors, crap pads, etc,. You may have insufficient vacuum for the brake booster, do you have a radical camshaft?

 

It was a bad m/c ended up blowing the seals in to the res. put a new one on no more problems.