Q: Why is there air in master cyliner on 1993 GMC 1500 Pickup

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I have repalced both rear brake cylinders, springs on one rear wheel,both front clynders, replaced the master cylinder twice and brake booster twice. I have bled the brake system numerous times. I still do not have any pedal. Most of the time I can have good hard pedal with engine off, but when I crank the enginge the pedal goes to the floor.When you push the pedal with engine running you can see huge amounts of air coming up in the master cylinder.
(1) Answer
If you didn't bleed the master cylinder on the bench to get the air out, that's one thing you can try. I'd never put in a master cyl without bleeding it in a vise on the bench. It's also possible that you have a defective master cylinder, especially if it was a rebuilt unit.