I have a 92 chrsy. 5th ave with no antilock brakes. just base brakes with rear drums. replaced master cly. due to leaking and replaced all the brake lines and hoses. i have good streams of fluid from all 4 wheels. and a solid pedal when car is off. soon as i start the car. brake pedal goes to the floor. rebled brakes, replaced 4 master cyls., both front calipers, wheel cyls., prop. valve, brake booster and check valve. bled brakes with a power bleeder, by pumping the pedal, and gravity bled. any clues!!?!?

If the pedal drops to the floor without you stepping on it or with very little effort then i would say bad booster or bad check valve on the vacuum line going to the booster. Hops this helps.

When you bleed the brakes are you starting with brake furthest from the master cylinder? Also was the master cylinder bench bled? If everything there is okay and you are not getting any air in fluid stream, you could try bleeding the brakes by having some one slowly pump the pedal till it is firm. Then have them let off open first bleeder valve and slowly depress to floor and hold there untol the valve is closed. repeat this at all bleeders. You may want to have car running to do this. Try this and let me know.