Question about 2002 Saab 9-5

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I have a 2002 SAAB 95 Aero that was running fine but developed a leak in the brake booster (made a hiss until you touched the brakes). I had the booster replaced and now it is quiet until you touch the brakes and then you hear a faint hiss. After I had it replaced I heard a very faint knock (or ping) under very light acceleration and took it back to the dealer. They said they couldn't hear it. My wife said it sounded more like something was loose. I gave up and just tried to avoid the conditions where I heard the noise. Recently I was accelerating from around 20 mph to join traffic and heard loud pinging and saw a small amount of smoke from the tailpipe. It went away when I backed off the throttle. Normally it seems like the car runs fine (although I may have adjusted to drive 'nicer') but if I'm accelerating hard from 15 to 20 mph I'll get a knock. I'm going to make an appointment with the dealer to get it checked but would like to have some idea what is going on. All I can think of is a vacuum leak.

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Check the ac vacuum routing also since vacuum lines are attached to that. You may have water in your fuel as well causing the pinging. Add a can of HEAT to the fuel tank

Posted on Aug 28, 2010

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Power booster seems to leak how to fix


These are not rebuild-able, they must be replaced if they leak.
One that leaks will hiss while the engine is running. The hiss is vacuum leaking, not pressure leaking out.
To possibly check it under the hood, be very suspicious of a leaking vacuum line at or near the booster, not the booster itself. It might be bad, but that is pretty rare.
The last one I changed I thought was defective because it hissed when I applied the brakes, but a new one did the same thing! :-(
If you decide to replace it, you will be unbolting the master cylinder from the unit under the hood, and also unbolting the unit under the dashboard.
Of you are really careful, you might not need to disconnect the master cylinder 's brake lines at all, just gently pull/push it out of the way to change the booster.
Be really careful not to kink the brake lines.
The unit is bolted to the car with usually 2 bolts through the firewall behind the brake pedal.

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hissing noise is a leaking diaphram inside power brake booster.you have to replace the power brake booster.

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A hissing sound when depressing the brake pedal is an indication that the booster is leaking. Another indication is that the pedal will be more difficult to push down before starting the car after it has been sitting for a while. A faulty brake booster will only make it more difficult to apply the brakes. However, it should be noted that a leak in the booster may be caused by brake fluid leaking from the master cylinder into the booster and this could become a dangerous problem.

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Hissing is more associated with a failing brake booster. Since it uses manifold vacum developed by the running engine, you will hear a hiss as the engine tries to restore the lost vacum.

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1 Answer

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It sounds like the rubber diaphragm in the brake booster has developed a leak producing the hissing / sucking noise. The only option is to replace the booster as a complete assembly as they need specialist tools to repair. If the booster fails completely you will still have brakes but the pedal will be very hard and it will take a lot of muscle power to stop vehicle. Best to have booster replced soon before you experience situation described above.

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Hi if you have a hard pedal, i would suspect you have an air leak on the servo pipe which goes to the brake servo, this is the same pipe that the brake booster is attached to, if you run the car and listen you should be able to hear the air hissing out from somewhere, hope this helps Chris

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1 Answer

95 gmc sierra 4X4 brake pedal very hard to push. every time touch brake pedal rpms go up 500, and it hisses on the inside of the cab. any solutions?


I think I would be looking at the vacuum booster for the brakes having a leak. Block off the vacuum line and try the brakes again. Make sure you have lots of room to stop. If the booster is bad then the pedal will still be hard but the RPM's and hissing should go away. Hope this helps.

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