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Power drum brake problem
I'm having a strange problem with my power drum brakes that I'm hoping the forum can help me solve.
It's on my 66 Satellite with the original single circuit braking system. After the car has been running for about a half an hour, the brakes on all four wheels start to drag. When this happens, the brake pedal doesn't seem to get any harder, but it does "build up". By that I mean I don't have to push it down as far to apply the brakes. If I let it go until it's inevitable end, the brakes will lock up, I won't be able to push the pedal down at all and the car will be unable to move (generally happens in the drivethru of the local Choke "N" Puke). If I crack the fitting on the output of the master cylinder there will be pressure inside. Releasing some of that pressure will make the problem go away for the rest of the drive only to return the next day. I have changed the MC twice and had the brake booster rebuilt (although they said there was nothing they could see wrong with it). I've checked about everything I can think of that could cause it. I thought that perhaps the pushrod into the back of the MC might be too long and not allowing the piston to retract far enough to uncover the port inside. To test that theory I drove the car until the brakes began to drag, then I stopped and unbolted the MC and pulled it away from the booster. It made no difference whatsoever. I thought it might have something to do with the lack of vacuum from the engine (about 6 inches at idle). Except that before the brakes begin to drag or after I release the pressure, the brakes function normally, with normal brake pedal pressure. It's got me baffled. Does anybody out there have any ideas? |
#2
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Take a look at your self adjusters. The Star key, at the bottom of the drum set up. Make sure the move freely and are installed the correct way.
The springs should be replaced if they are old. Thats all I can think off. |
#3
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poportioning valve might be sticking. have you tried bleeding your brakes ?
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#4
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Just a thought.How close is your master cyl. line to your exhaust or headers if you have them?
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#5
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I was under the impression that there is no proportioning valve with a drum system. I know there is a distribution block on the frame rail where the rear line and the two front lines branch off but I don't think it does any proportioning function. I could be wrong.
There's probably 8 or 9 inches between the MC and the header and none of the brake lines run close to the exhaust. The brake hardware is relatively new -- probably doesn't have but a couple thousand miles on it. I don't think the adjusters could be the problem. If it was, once the adjusters caused the brakes to drag the dragging wouldn't go away. The way I see it, when I crack the fitting on the output line from the MC and bleed off some pressure and the brakes release, that tells me that everything downstream of the MC output port is working just fine. There has to be something either in the MC, booster or pedal linkage that is causing the MC to build pressure in the system and not release it. I just can't figure out what it could be. The linkage is stone-age stuff, nothing high tech here and isn't rusted or sticking. I've had the booster rebuilt so, if the brake shop is to be believed, it should be fine. I've changed MC twice so surely all three of them couldn't be bad. That's why I've been kind of wondering if it couldn't have something to do with the vacuum, or lack thereof. But the symptoms aren't what I would expect and without spending lots of money I'm not sure how to go about troubleshooting that. |
#6
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Youve done everthing I would have
and its usually something so simple that its easy to overlook.When my harley was doing the same thing I had to open the return hole in the master a little and it was fixed.So make sure that the piston in the master is returning to full rear position and you should be able to see it squirt out when you release the pedal.If not remove the piston assy and open it up a little with a small drill,make sure there are no burrs on the inside of the bore and reassemble.Might just need a heavier return spring on the pedal.
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#7
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There should be a slight amount of pressure trapped in the line coming from the master cylinder, that's what the Residual Pressure Valve (RPV) in the master cylinder does. The residual pressure is usually in the 2-8 psi range and the return springs at the brake shoes can easily overcome that.
If there is sufficient pressure being trapped in the outlet line to hold the brakes, either the RPV is faulty or the MC is the fault. Even though loosening the MC from the booster doesn't relieve the pressure, the MC could still be at fault if the piston isn't returning sufficiently to expose the return port. The fact that the pedal "builds up" confirms this. With the MC cover removed, depressing the brake pedal should produce a burble in the surface of the fluid as the piston seal is closing the return port; if it does, try it several more times to be sure it repeats. Even though the MC has been changed, I'd still suspect it as the problem. |
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