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

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.
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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.
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