vaccum brakes


Active Member
Jul 7, 2008
i wanted to know if it is possible to have a vaccum leak inside the brake booster. the car idles pefectly fine in park and while crusing but as soon as i press the brake it starts idel hunting. the idel goes up and down and the car sometime shuts off. i changed the vaccum lines the go from the booster to the vaccum block.
Not sure if this will help:
Once you've determined a vacuum leak is present, here's the easy way to locate it. Get a piece of 3/16" rubber vacuum line several feet long. Carefully place one end to your ear and move the other end around in the intake manifold area. The hissing sound of the leak can be clearly heard through the tubing and will guide you right to it.

from: Diagnosing Engine Miss
No leaks around the intake manifold? The next most likely suspect is the power brake booster. Pull the hose off with the engine idling and plug it with your finger. If the miss goes away, the diaphragm in the booster is leaking or the vacuum hose connection to the booster is faulty.

from: Troubleshoot Power Brakes
From this, it should be obvious that the vacuum booster needs two things to do its job: a good vacuum supply from the engine, and a good diaphragm. A vacuum supply hose that's loose, leaky, collapsed or restricted may not allow the booster to receive enough vacuum to provide the usual amount of power assist. Consequently, the driver will have to push on the brake pedal harder to get the same braking as before.

A restricted vacuum hose will cause boost to drop off when the brakes are applied in rapid succession. This happens because the blockage slows the return of vacuum in the booster.

To check engine vacuum, connect a vacuum gauge to the supply hose that runs from the intake manifold to the booster. A low reading (below 16 inches) may indicate a hose leak or obstruction, a blockage in the exhaust system (plugged catalytic converter, crushed pipe, bad muffler, etc.), or a problem in the engine itself (manifold vacuum leak, bad valve, head gasket, etc.).

The condition of the diaphragm inside the booster is also important. If cracked, ruptured or leaking, it won't hold vacuum and can't provide much power assist. Leaks in the master cylinder can allow brake fluid to be siphoned into the booster, accelerating the demise of the diaphragm. So if there's brake fluid inside the vacuum hose, it's a good indication the master cylinder is leaking and needs to be rebuilt or replaced. Wetness around the back of the master cylinder would be another clue to this kind of problem.

To check the vacuum booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit. Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. No change? Then check the vacuum hose connection and engine vacuum. If okay, the problem is in the booster and the booster needs to be replaced.

Vacuum boosters also have an external one-way check valve at the hose inlet that closes when the engine is either shut off or stalls. This traps vacuum inside the booster so it can still provide one or two power assisted stops until the engine is restarted. The valve also helps maintain vacuum when intake vacuum is low (when the engine is under load or is running at wide open throttle). You can check the valve by removing it and trying to blow through it from both sides. It should pass air from the rear but not from the front.

Replacing a vacuum booster is a fairly straight forward job. All you have to do is disconnect it from the brake pedal on the inside and unbolt the master cylinder. The pushrod that runs from the booster into the back of the master cylinder must have the specified amount of play, so check your service manual for the particulars. Most require a small amount of play so the master cylinder will release fully preventing brake drag, but some late model GM and Bendix applications have zero play.
I had a "G" body Malibu that had the same problem. Turns out the rubber bladder inside the brake booster was split and wouldn't hold pressure. As mentioned before look else where for a possible vacuum leak first. If you think you find one just spray some WD40 at it and if the RPM increases you got a leak.
I went through 3 vacuum boosters (lifetime warranty) & finally went with hydroboost.

I still want to know how GM was able to make the TA work without these same issues?
will this cause the symptoms i posted?
so far all symptoms are leaning towards a bad booster

Yep! Most likely the Air Valve is leaking and not the diaphram. The only fix is to replace the booster.

Plenty of threads in the Brake and Suspension forum on the difference between the different boosters.
Do your brakes seem harder to work (require more pedal effort) for normal/typical braking?? Hows about performance/effort building boost off foot-brake??
Vacuum uses engine vacuum to assist the brakes.

Hydroboost uses PS pump pressure to assist the brakes.

PM uses an electric pump to assist the brakes.

Vacuum has least amount of boost which is not available when the turbo is providing + pressure, hence no boost (don't ask how I know).

I believe the hydroboost provides more boost than the pm. Hydroboost was on 84 & 85 GN & on many new cars & trucks today (especillay diesels).

Pm is probally the most expensive to repair, however, hb is close on expense to install.
Do your brakes seem harder to work (require more pedal effort) for normal/typical braking?? Hows about performance/effort building boost off foot-brake??

yes brakes seem alot harder. and when i try to build boost the car starts creeping forward.