hissing vacuum booster - PeachParts Mercedes-Benz Forum

JHZR2 · #1 02-25-2003, 12:58 PM

Hi,

When I shout ff my car, I always hear a hissing type noise from inside the vacuum booster by the master cylinder. The brakes are OK for one pump within about a day, but after that, the pedal is hard. My other cars dont seem to make a noise, and the pedal will give vacuum assist even after a week of sitting.
One of the nipples on the line going from the vac pump to the brake booster was broken off since I got the car, but it has been plugged, and vaccum problems have only really been apparent lately. This noise has been apparent for a long time as well, but I might as well get to the bottom if it now, while Im at it.

Any info on the normality of this sound is appreciated.

JMH

lrg · #2 02-25-2003, 03:39 PM

It's not normal and you obviously have a vacuum leak. Check the rubber vacuum tube connectors around the top of the oil filter canister. These are easily knocked off when changing the filter. If that's not it I'm afraid you just have to work your way through the vacuum system until you find it. Unfortunately these kinds of leaks aren't easy for us to help you find by talking you through it, you just have to listen for the leak and/or test the system until you find it. Good luck!

JHZR2 · #3 02-25-2003, 03:42 PM

yeah, I know diagnosing these leaks is rough... and Ill have to do it. But, the sound is definitely coming from inside the booster, the connections are all tight, and the broken nipple i ssealed, and by listening, I can tell that the broken nipple isnt the culprit...
Thanks

JMH

lrg · #4 02-25-2003, 03:48 PM

If you are sure the hissing is coming from inside the booster then it may be that the diaphram inside is bad. I'm not sure if they can be rebuilt on these cars (they can on some) but if not a good used one shouldn't be very expensive.

Fisherman · #5 02-25-2003, 05:38 PM

Could be too that what you hearing is a sympton and not the problem...
Trying to think positive here...
Could be a leak somewhere else inthe system and you cna just hear the pressure releasing in the can. The other vacuum resevoir that is close to this location is the one in the fender. If you have a mity vac or can borrow one you can check the lines to the two vacuum resevoirs pretty easily from the firewall connections.

What about a check valve that is leaking too allowing the vacuum in the booster can to bleed back into the main system? Try pulling out the check valves and then see if it leaks after building vacuum again.
Okay, I;m fishing here....

jhal · #6 02-25-2003, 08:28 PM

It could be the master cylinder to brake booster seal (o-ring I think,maybe).......fishing also, but worth a check.