Bleeding breaks in a 1996 Infiniti I30

[truelove repair service]

Anyone have an idea what could be causing pressure to not build properly in the break system. Blead each of the breaks. Pump until pressure and then release the bleeder valve. Lock down and repeat several times. Do same for all 4 brakes. Brake goes to the floor. Pump it several times and it builds pressure. Can see brake fluid in master cylinder going down as brakes are pumped. It will hold the pressure. Once brake is released, fluid is returned to resevoir and again no pressure. Must pump. Bleed all brakes again. Same thing. This is the third master cylinder installed. No leaks in lines. Any ideas what might be happening here?

[tigersharkdude]

check your calipers, when I painted my calipers, I found out my passenger rear caliper was busted