I had an air problem with a 72 corvette. Tried everything. New calipers, master cylinder, new lines. Finally after 2 or 3 years I gave in 7 took it to a corvette shop. They put on new rear calipers. The old ones were sucking air. The old ones I bought new from a brake supplier. They never worked from day one. The clue on mine was that they had fluid at the bleeder and after pumping the pedal they went dry. Now my brakes are fine.