Brake Vacuum. Problem?

[hendo#1]

How long should a brake servo hold / store vacuum?

Driving along today at normal speeds I brake for a car turning right in front of me. No problems with braking. Once the car has turned off and is out of my way I gun it through 1st, 2nd and into 3rd.

At this point I notice a farmer potentially about to pull into my path out of a gateway so I hit the brakes.

For a moment, there is no / very little assistance. Upon rapidly releasing the pedal and re-applying (once), full braking force is achieved.

Is this normal, or have I got a split somewhere leading to the servo?

In normal driving I spend a lot of time in vacuum, so it may have always been like this, and only now, when requiring the brakes after a full boost run have I noticed it.

Thoughts?

Edit to add - BAM, 2003 S3. It still retains the original 1 way valve at the bulkhead, plus the additional one I installed months ago at the inlet mani.

 

[S3 Hilife]

How long should a brake servo hold / store vacuum?

Driving along today at normal speeds I brake for a car turning right in front of me. No problems with braking. Once the car has turned off and is out of my way I gun it through 1st, 2nd and into 3rd.

At this point I notice a farmer potentially about to pull into my path out of a gateway so I hit the brakes.

For a moment, there is no / very little assistance. Upon rapidly releasing the pedal and re-applying (once), full braking force is achieved.

Is this normal, or have I got a split somewhere leading to the servo?

In normal driving I spend a lot of time in vacuum, so it may have always been like this, and only now, when requiring the brakes after a full boost run have I noticed it.

Thoughts?

Edit to add - BAM, 2003 S3. It still retains the original 1 way valve at the bulkhead, plus the additional one I installed months ago at the inlet mani.

I wouldn't run two inline check valves to the servo, I made the same mistake a while ago and found I intermittently lost servo assistance as there was a delay in both valves opening on vacuum




Steve

 

[hendo#1]

Thanks Steve, sounds logical.

I will give that a go before I start pulling pipes etc out.

 

[ant schof]

Check the elbow connector by the servo , found mine was completley split last year 2002 S3 ,

Cheers Ant

 

[AGB-S3]

For a moment, there is no / very little assistance. Upon rapidly releasing the pedal and re-applying (once), full braking force is achieved.

Join the club.. I have the same thing , never yet got to the bottom of the situation....diesels have a VAC pump , so are immune from this , personally, I'd say its a grade one **** up , using inlet vac , drawn from a system that spends most of its time under positive pressure , even my old xr4x4i had a pumped breaking system .

Tempting to suspect that , boost pressure is reaching the servo unit , the other is the servo unit leaks , I only have servo assistance for the brakes , for a short time, with the engine stopped , which points to a leak in some part of the system .. the pipes and fitting , by now will be on the verge of perishing , changing the formed pipes , did result in a smoother tick-over and improved, but not perfect braking

G ,

 

[Erikn89nl]

Soon as you let off the accelerator the butterfly valve should mostly close, causing a vacuum in the intake manifold which operates the DV and should provide vacuum to the brake servo. If your DV works as it should but your brake assistance doesn't, IMO this points to a fault in the brake servo or associated pipework...

My A3 (1998) had several splits in the plastic pipework that runs from the intake mani to the servo, mostly on the joins. I replaced with an appropriate size reinforced silicone hose (remember to fit the check valve, and put it the right way round) and it's been fine ever since

 

[AGB-S3]

There are brakes , for a short time with the engine off , the issue comes with a need to stop , during a period of acceleration .. during which time, brake vac appears to of leaked .... difficult to get a definitive description of what should happen... other than the servo can be suspect ....

 

[Erikn89nl]

Soon as you take your foot off the loud pedal, brake vac should be back, unless you are left foot braking... I'd check the pipework and check valve for leaks first off.

 

[AGB-S3]

Q with the engine off , how many sequential times can you operate the brake, before the vac fails ? on/off/on / off ?

 

[hendo#1]

Right so as per S3 Hilifes suggestion, I did away with the additional 1 way valve I installed (after simplifying the PCV system - conflicting advice online). The S3 does indeed have a 1 way valve at the bulkhead, an additional one is not needed.

Anyways - no difference.

So I pulled out the pipe that feeds the brake servo and found a split pipe. Apologies for the very very poor pictures.

The split is after the one way valve at the bulkhead. Meaning boost was not leaking as it wasn't getting past the valve, but it was leaking on vacuum readily and easily.

The pipe can be split at the rubber joint so you don't have to replace the whole thing if the rest is in good condition. This pipe for the servo side was 8L6 612 041G @ £37.03+VAT. I should have it for Tuesday and will report back.

I suggest everyone has a look at theirs. Run your car so it starts boosting then immediately hit the brakes. If you have no servo assistance momentarily, you probably have a split like this.

 

[hendo#1]

New pipe installed, problem sorted.

 

[AGB-S3]

can you do the engine off , brake check ? , just see how may times , you can press and release the brake, with the engine off , till the VAC fails , ie, the peddle becomes ''solid'' ??

Tnx-Graham

 

[Brad1011]

I think I have this problem will check it the weekend cars elsewhere, anyone else experienced this, Ever since I’ve had the car brakes have been awful I put braided lines on re bleed thought master cylinder had gone replaced that still the same brakes work.. but not great pedal travels all the way down then I feel the brake there’s no pressure there at all. I thought I needed someone with vagcom to do a abs bleed I’m guessing this is quite common


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[Brad1011]

I also have prefacelift s3 1999 which I’m guessing they changed the vac line hence the new part number ?


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[AGB-S3]

After having exciting brakes for quite some time, ie virtually none after long time at high speed ... with very short vac retention, had even thought of fitting a boost pump .. finally , just by luck, was found to be a vac connection ''rubber'' hose , had been replaced with what looks like a plastic house with fake cloth pattern ..this was leaking

fitting a new hose assembly , with vac valve to the brake servo ,
cured the problem , brakes worked much better and hold vac over night
later other vac lines where also found to be split

the car now idles smoothly with the air-con on , something it failed to
previously .... [probably a good test ]

G

 

[Brad1011]

After having exciting brakes for quite some time, ie virtually none after long time at high speed ... with very short vac retention, had even though of fitting a boost pump .. it finally , just by luck, was found to be a vac connection ''rubber'' hose , had been replaced with what looks like a plastic house with fake cloth pattern ..this was leaking

fitting a new hose assembly , with vac valve to the brake servo ,
cured the problem , brakes worked much better and hold vac over night
later other vac lines where also found to be split

the car now idles smoothly with the air-con on , something it failed to
previously .... [probably a good test ]

G

Sounds promising I’m going to check the pipes out the weekend I feel kinda excited my idle also moves up and down not a lot though


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