79 150 brake help please
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79 150 brake help please
My 79 has a curious brake problem. No brakes...replaced booster, M/C (bench bled), front pads, rear shoes. Bled the system back to front. Excellent pedal with the engine off. Pedal not so slowly goes to the floor with the engine on, Have to pump the brakes to get the truck to stop. No leaks. Any suggestions would be appreciated.
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Can you describe the procedure you used to bleed the brakes? I know you said no leaks, but if the pedal is sinking, you have air in the lines. So you're saying that the pedal sinks with the truck on, but is restored when it's off?
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thanks for the replies. I bench bled the mc. I then bled right rear, left rear, right front, left front (adding fluild as needed to keep the m/c from going empty). Bleeding procedure is pumping the brake with bleeder valves closed, holding the pedal, open the valve to let air/fluid out, closing the valve, then releasing the pedal. repeating until only fluid flows (no air coming out).The pedal is great when bleeding (started low when bleeding procedure started) got progressively better as procedure commenced. Full pedal when the engine is off, but engine on it goes to the floor (have to pump pedal just to get truck stopped) when just driving around the block.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
That's interesting. Your procedure sounds dead-on. Sounds like something to do with the booster. With a healthy booster, when the engine is off, you get a few good pumps before the pedal stiffens up because there is no longer any vacuum applied to the booster. That's a quick and dirty way to tell if your booster diaphragm is ruptured. The booster is a pain to replace because you'll have to take off your MC again and bleed the brakes again when everything is back on. Boosters are fairly expensive (~$100), there is a core charge, and it's confusing getting the right one because of changeovers between serial numbers. As such, the booster isn't something you want to throw parts at until you're sure it's really the problem.
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