Brake Booster?

clinehall · **Posted:** Sun Jul 31, 2011 3:24 pm

I have constant hissing from my brakes, except under braking. There is no difference in braking. Is it the booster or just a hose or something. BTW, it is on my ML320

clinehall · **Posted:** Sun Jul 31, 2011 4:02 pm

i guess I could hook up a mighty vac to the hose and see if it holds pressure.

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Where is the hissing coming from, and how do you know it's coming from your brakes?

If you have a vacuum leak *somewhere* in the engine, I imagine that pressing the brakes could change the sound of the leak, since the booster is effectively another big leak.

If your brakes feel normal I'd look for vac leaks elsewhere. Unless you have something that points you to the braking system specifically. If the sound is from under the dash, then there's a good chance something is going on with the booster.

Re the mityvac -- if your brake pedal stays "soft" feeling and your brakes work great once or twice AFTER shutting the car off and sitting for a while, the booster isn't leaking. If the brakes get "hard" (loose the existing vacuum quickly), that might point to a leaking booster. Could be other things too (eg leaking check valve)

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Let's clarify a bit. When you say "hissing" is this definitely a "vacuum leak" kind of hiss, or could this be a light brake drag? It is quite common to have a brake caliper that is not fully retracting after brake release and that can give a sound that can sound a little like an air leak. In my experience, a vacuum leak loud enough to be heard without really trying to listen for it (using a piece of hose or some other amplification device) is likely to cause drive-ability issues. If it is a dragging brake, a light touch of the brake pedal makes it go away.
Unfortunately my truck has this problem now. I replaced the caliper thinking that it was the culprit, but it seems that I have a swollen rubber brake line that is holding a little pressure on the pads.
Charlie G

RichardNuss · **Posted:** Thu Aug 04, 2011 5:26 am

Charlie Guthrie wrote:

Let's clarify a bit. When you say "hissing" is this definitely a "vacuum leak" kind of hiss, or could this be a light brake drag? It is quite common to have a brake caliper that is not fully retracting after brake release and that can give a sound that can sound a little like an air leak. In my experience, a vacuum leak loud enough to be heard without really trying to listen for it (using a piece of hose or some other amplification device) is likely to cause drive-ability issues. If it is a dragging brake, a light touch of the brake pedal makes it go away.
Unfortunately my truck has this problem now. I replaced the caliper thinking that it was the culprit, but it seems that I have a swollen rubber brake line that is holding a little pressure on the pads.
Charlie G

And if it is the booster itself hissing (or most other vacuum leaks), it should do it even when the car is not moving, right? While a dragging caliper or something like that would only make noise while the car is in motion.

clinehall · **Posted:** Thu Aug 04, 2011 10:03 am

It hisses very loudly (in cabin) until the brake is used then it doesn't and brakes work 100% fine. I was going to go ahead and put a booster on and see but I can't find one under 583 (and not sure if that is really available. Dealer wants 729. Apparently the company the does the rebuilding does not have any to rebuild. I have found a salvage for 244, might have to resort to this and send mine to be rebuilt.

I haven't worried about it but I am letting my daughter borrow the car to drive to Florida and don't like the idea of it with out me driving the car.

Vincent Keene · **Posted:** Thu Aug 04, 2011 10:21 am

clinehall wrote:

I haven't worried about it but I am letting my daughter borrow the car to drive to Florida and don't like the idea of it with out me driving the car.

Where was this car built again?

clinehall · **Posted:** Thu Aug 04, 2011 10:52 am

Vincent Keene wrote:

clinehall wrote:

I haven't worried about it but I am letting my daughter borrow the car to drive to Florida and don't like the idea of it with out me driving the car.

Where was this car built again?

Tuscoolusa County, Alabama BAZINGA :twisted:

http://mbusi.com/

jeremy gast · **Posted:** Thu Aug 04, 2011 11:30 am

Cline you think that is any better than Germany???? :stick:

clinehall wrote:

Vincent Keene wrote:

clinehall wrote:

I haven't worried about it but I am letting my daughter borrow the car to drive to Florida and don't like the idea of it with out me driving the car.

Where was this car built again?

Tuscoolusa County, Alabama BAZINGA :twisted:

http://mbusi.com/

Vincent Keene · **Posted:** Thu Aug 04, 2011 1:31 pm

clinehall wrote:

Vincent Keene wrote:

clinehall wrote:

I haven't worried about it but I am letting my daughter borrow the car to drive to Florida and don't like the idea of it with out me driving the car.

Where was this car built again?

Tuscoolusa County, Alabama BAZINGA :twisted:

http://mbusi.com/

Even though the Z3 is built in South Carolina and I wouldn't buy one of those either. Hans is lurking somewhere in that plant too.

clinehall · **Posted:** Thu Aug 04, 2011 2:22 pm

jeremy gast wrote:

Cline you think that is any better than Germany???? :stick:

Where was this car built again?

Tuscoolusa County, Alabama BAZINGA :twisted:

http://mbusi.com/[/quote][/quote]

NO I think it's worse, haven't you been through Alabama? I was just waiting for VK to go there and I was pointing out that all things that are crap are not german.

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