Brake Booster Hiss

Coog

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On a part press of the brake pedal in the MK2 Golf I get a hissing noise and the engine idle stumbles... so I have an air leak. The hissing is coming from the seal surrounding the push rod behind the brake pedal. On full press it disappears.

I replaced the master cylinder along with pretty much all other parts of the brake system a few months back and this is the first time testing it. The servo was the original '92 part which is untested until today.

If I pump the brake pedal with engine off I get a solid pedal that'll hold.

Could a crap / poorly fitted master cylinder cause this or do we reckon it's just the diaphragm in the booster is gone? I can't think why the MC would cause it but thought I'd best check. Servo's aren't cheap to replace unfortunately.
 

stevieturbo

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a better test for leaks...other than an actual vac test with a mityvac or similar. After it's stood for a few hours, is there still vac assistance or has it gone hard already ?
 

Coog

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The pedal is hard with the engine off after a couple of pumps. The moment the engine is started the pedal drops very quickly. It pretty much drops to the floor.

Haven't tried it after a few hours, will give that a go and see what happens. There's definitely air rushing through pedal side and it'll run like there's a pipe off with the pedal half pressed.
 

Coog

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That means it's working

Any ideas what would cause hissing in the cabin and idle to drop? I've never had one of these off or apart, so up until now had no idea how they worked.
 

Gambit

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You switched the pierburg auto choke to a Weber manual choke? The auto choke uses vacuum doesnt it? The big green ball, how did you block of it?

Just trying to think of things close enough to bulkhead that you could hear it in cabin
 

Coog

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Yeah all the pipework seems dead on. I did away with the reservoir altogether and blocked a couple of pipes off. The vacuum from the manifold seems OK though the seal to the manifold itself is a bit weak so that'll need fixed. It still runs grand though until the brake pedal is depressed so the act of moving the pedal is introducing a vacuum leak.

I'm almost certain it's hissing through the rubber seal / sponge filter at the brake pedal. I can change the noise by grabbing and moving the seal around.

Could it be a knackered vacuum valve?

I can't understand why the pedal would stay hard with the engine off.

Flipping cars lol

Screenshot_20200530_070807.jpg
Screenshot_20200530_070739.jpg
 
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Nicky Glanza

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I changed a servo there few weeks back and got a similar issue with brake pedal being solid with car off and spongy with the car on, I bled the brakes up 3 times before getting all air out and circulated around 2.5 ltr of brake fluid to get the air out now it’s perfect.
I would also check the seal on the engine side of the servo your vacuum hose comes out of, boys have a wild habit of using a small flat head to pop them out and sometime a wee nip will cause the issue as well
 

Coog

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Wasn't tested on the pierburg unfortunately. I'll go back over the plumbing again and check it over.
 

stevieturbo

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I can't understand why the pedal would stay hard with the engine off.

if there is no vacuum available, the pedal will be hard. That's always the case. So f it's going hard almost immediately, vacuum is leaking from the servo somewhere.
 

Coog

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Ok pulled master cylinder and got a woosh of air as the seal broke on the booster.

Checked vacuums and all seemed good.

Still getting a hard pedal after a few presses but noticed that with the engine off I get a hiss inside on first press of the pedal.

Oh and no hiss on hard push of pedal, just if you're braking gently.

Got a video:

 

Coog

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Yes valve still there and seems to be working ok but didn't want to remove it to test. Reckon it could be knackered?

Screenshot_20200531_153422.jpg
 

NI_Volvo_Nut

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If you are getting a hissing sound with partial press of the pedal its the seal around the push rod. Kits can be got on ebay, might be easier to just get a new servo though.
 

Coog

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Defo worth checking even just to rule it out, what the other wee connector below it for?

That was for the pierburg. The hose really only goes from manifold to the servo and also to the advance on the distributor. That's it really.
 

Coog

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If you are getting a hissing sound with partial press of the pedal its the seal around the push rod. Kits can be got on ebay, might be easier to just get a new servo though.

Internal to the servo? Pedal side should be at atmosphere, there shouldn't be any air rushing through there to the engine, right?
 

NI_Volvo_Nut

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Internal to the servo? Pedal side should be at atmosphere, there shouldn't be any air rushing through there to the engine, right?
You raise a valid point and in theory I would agree - however - I have experienced this mainly on Volvo's S60 & V70 which is an engine removal job to get the servo out. Someone came up with a seal replacement kit which can be done in situ, I have since seen this kit listed for various other cars as well. Now I think about it that probably means a ruptured diaphragm inside the servo. Either way there shouldn't be any hissing in the cabin and the hissing is the vacuum escaping and therefore allowing extra unwanted air on the intake side of the engine which will be causing idling and more than likely running issues.
 
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