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No, not that valve... (archive)

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Posted by (e.g. brake booster line) on June 29, 2001 at 15:55:59:

In Reply to: Re: Hard brake pedal feel posted by john on June 29, 2001 at 15:14:43:

It's been a while since I took apart an 02 booster, but I recall that they were a lot like standard American car boosters, whose design really hasn't changed much since the late '50s.

Inside the booster there's an actuation mechanism. When the input shaft or "trip rod" is moved forward by the pedal linkage, a valve gizmo (not sure of the proper name) opens to allow an imbalance of pressure of one side of the diaprhagm relative to the other. The way this often works is that, when your foot is off the brake, both sides of the diaphragm are exposed to manifold vacuum (via the one way valve, so that flooring the throttle does not cause a drop in brake vacuum). When you press the brake, the back side of the diaphragm is exposed to atmospheric pressure when the valve gizmo opens. This pressure imbalance causes the diaphragm to push forward. Letting your foot off the brake closes the valve to the atmosphere, and opens a valve between the chambers, so vacuum again builds up evenly on both sides of the diaphragm. On the 2002 unit, a heavy coil spring inside the booster pushes the diapragm backwards to help release the brakes. When the booster doesn't work, you are not only fighting normal non-boost brake pressure, you are counteracting this return spring as well.

If the diaphragm develops a leak, then there will be no vacuum assist to the brakes, and your car will run like crap when you step on the brakes--this is because the valve gizmo lets air into the back of the vacuum chamber, but the diaphragm leak lets the air travel all the way up to the manifold.

Another failure mode, the actuation valve gizmo becomes plugged so that the backside of the diaphragm is not exposed to atmospheric pressure when you press the brake.

These vacuum units are not designed to be rebuilt; the halves are stamped together. That is really the only thing preventing rebuilding. Systems on some very old cars ('50s American luxury cars) were designed such that the two halves of the booster could be separated, and the internals cleaned up.

If the booster is acting up, you could either replace it (but if you replace it with an old one, sooner or later it will quit as well) or attempt to rebuild it. If you choose the latter, you will probably need to redesign the can so that the two halves can be joined by screws, using a large O-ring as a seal. Some shops advertise that they can rebuild these things to factory specs (see the Services section in Hemming's Motor News), but longevity after the rebuild is anybodys' guess. Rebuilt ones are prone to vacuum leaks, because the stamped perimeter seal doesn't take kindly to being opened and re-stamped.


do you mean the little one-way valve in-line in the vacuum hose? I hope not. I have the same symptoms (I've learned to just live with it). And that little valve is fine.

You didn't damage the unit by installing dual 40's. Also, if the vacuum unit is leaking, your car will run like complete crap--especially with sidedrafts, because the vacuum port affects only one cylinder. It sounds like the vacuum unit control valve is stuck.


I haven't been driving my '02 for a while (just replaced the head gasket) and now I have a hard feeling at the brake pedal. It feels as if the power assist isn't working. If I start the car, the brake pedal does not get easier to push. The vacuum hose is hooked up to the manifold, and if I disconnect it from the assist I can feel that it is sucking like nobody's business.

Is there anything I should check before I replace the vacuum assist unit? I recently put dual 40's on and am wondering if that caused more vacuum and could have damaged the unit.

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