I recently had a problem with my brakes on my 1988 Chevy K1500. After replacing the drums, pads, and all the springs the brakes would not stay firm. We then replaced the master cylinder and power booster. Now the brakes will get firm when the car is off, but when the car is turned on the brakes will slowly go to the floor. I have bleed the brakes, and completly replaced all the fluid from all four tires. I am just wondering if you have any ideas. Thank you for your time.

 

Check the adjustment on the rear drums. If the shoes are not touching thr drums with the pedal down, it'll give You a strange pedal feel.