Chevrolet Repair: Brake system, master cyl, calibers

Question
I have a 1989 Chevy 1 ton dually that has air in front brake system. I have changed front calibers brake lines and proportioning valve and bleed lines. When I start pumping the pedal it gets firmer but when I stop for a minute it goes all the way to the floor again. I have bleed the brakes for hours and still no luck. Is there anything I could possibly be overlooking?

Answer
Hi Sharon... Bleed the master cyl... you have air trapped in it. then rebleed the system after you get all the air from the master.also make sure that the centering valve is in the center while bleeding. (proportioning valve) hold the pin on the end in the center while bleeding after you get air from the master... hope this helps