Driving in Atlanta traffic can be trying at times. Sometimes I am forced to brake hard I hear a squealing noise from under the hood and the car stalls sometimes. My nephew says it might be the brake booster. Does this sound right? If so how hard is it to replace?

I'd rather not hazard a guess given only the info available so far. I'd doubt that a vacuum booster would have an intermittent leak. Think of it this way: In the engine compartment, in front of the driver, behind the master cylinder is the brake booster. There is a big fat vacuum hose that runs from the engine to it. Inside of it is a diapragm that aids you when you push the rod into the master cylinder (applying the brakes). What your nephew is implying is that the diaphram is leaking intermittently and that is allowing a ton of air to flow into the engine and lean it out so bad that it stalls (any air that enters an engine without going through the throttle body doesn't get 'measured' or 'counted' and therefore doesn't get a corresponding amount of fuel added for it).

I can't see how a vacuum leak would be intermittent. Typically, these things fail or they don't. If your nephew is correct, then you should be able to duplicate it anywhere, even when you are parked in your driveway.

A simple test that I would do if I were you is: Pull that big fat vacuum hose off while the car is running and in park (this is if you are by yourself). See if the engine acts the same way. Remember that more then likely you are introducing a %100 amount of air through the hose (unlike a leak in the diapragm) and therefore you have to take that into consideration. Perhaps remove the hose and cover the end with your thumb. Have someone else start the car and you slowly remove your thumb from the opening (this is if you have someone that can help you). That way the engine might get up to %50 or %80 without being a %100 full instantly. If you do that test and the car acts the same way, then I'd geuss that your nephew is correct and please post back or pm me so I don't continue giving out information that would be wrong.

There is another test that you could do (and I'd only suggest doing this in a large parking lot that is pretty empty (at a school on the weekend or a mall late at night)), since you won't have any 'assist' in applying the brakes (some of us remember what it's like driving without assist). You could try duplicating the problem until you know pretty much what makes the car stall for sure. Then remove that big vacuum hose that goes to the booster and plug it up (now you wouldn't have any vacuum assist and the pedal would feel hard). That would eliminate the possibility of a vacuum leak coming from the booster. Drive around the parking lot and try to duplicate the problem. If the problem is gone, then yes, the booster would most likely be the source of your problem.


I am sorry for this being such a long winded post. Unfortunately without much information to go on I find it best to give enough information to better diagnose a problem before someone goes out and spends a lot of money on something that wouldn't help. Please try and post back with what you find to be the cause, I certainly would like to know and it will help us help others (with similar problems) in the future.

Your response was not long winded at all. It is an excellent response. The leak really isn't intermittent. It only happens when I brake real hard. Then I hear a squeal that seems to me is coming from just on theother side of the firewall. Since that is where the brake booster is...

How difficult is the brake booster to replace? I am fairly handy with cars but I don't think I would try to tackle swapping an engine or anything.

I plan to implement your suggestions this weekend when the school parking lot near my home will be empty. Thanks again for the excellent suggestions!

Since you don't have to 'crack' a brake line it is a rather easy process.

First, unbolt the master cylinder from the booster (usually two nuts on studs), then try and move the master cytinder far enough forward that you can work without it being in the way (don't bend/kink any lines though). Then unbolt the booster from inside the car (under the dash) and remove. Of course, you will have to remove the pin from where the brake pedal attaches and remove the vacuum hose.

From what I remember (haven't done many, and pretty sure the ones I did do were not on Volvos), it was a simple 'remove-and-replace' job.