One for the brake gurus out there (NLC)

Mr Momo · Post by **Mr Momo** » Thu Jan 24, 2008 10:40 pm

OK, need to ask a straightforward question about the brakes on our 6y.o. Passat :

In the morning or after a period of non-use, the brake pedal is solid and the brakes have no bite at all, if you move off (read 'panic' as you try to avoid the car in parking space in front - we live on a slope). Thereafter, everything feels normal, apart from the pedal feeling slightly less smooth than normal.

Asked the dealer's service dept. to check this several times - first reponse was a leak in the master cylinder somewhere (replaced) - ok for a few days then issue returned. Then they rechecked fluids etc. with no change.

Garage then claims its a TADTS (at this time of year when moisture gets between pads and discs), which sounds like a bit of a cock and bull story to me.

What I get when I try to press the pedal into life is a sound like the fluid is 'squirting' (excuse the terminology) back into the reservoir. Once the initial reluctance is overcome, the brakes are fine and work normally.

Rear discs were replaced about 12 months ago and on the front a couple of years ago and look fine. Pads have plenty of depth.

Is this a concern ? Is there a solution I should seek ?

Thanks in advance.....

Rich H · Post by **Rich H** » Thu Jan 24, 2008 10:49 pm

Aside from the abs bollicks the brake system is simple hydraulcs. Does sound like master cylinder woes or something wonky in the ABS.

Either way the dealer is talking bollocks, if my Sheddy S1 and 1980's 924 don't suffer from it why would a 6 year old VAG??

Discs and moisture can cause sticking but not loss of brakes, both of my sheds start with a bang as they break free.

Mr Momo · Post by **Mr Momo** » Thu Jan 24, 2008 11:03 pm

RICHARDHUMBLE wrote:
Discs and moisture can cause sticking but not loss of brakes, both of my sheds start with a bang as they break free.

Agree with you there.

Too much moisture in fluid (it was supposedly changes a couple of services ago) ?

Air trapped ?

Rich H · Post by **Rich H** » Thu Jan 24, 2008 11:18 pm

Trapped air means spongey brakes not no brakes.

Change the fluid on spec?

fd · Post by fd » Thu Jan 24, 2008 11:24 pm

> side from the abs bollicks the brake system is simple hydraulcs

Eh . . . no - not quite . . .

solid pedal and no brake power means no power assistance . . . which means the vacuum servo is not working properly . . . I assume once things start working there is more movement in the pedal and also much more power ?

Some petrols and all diesels have a seperate vacuum pump and reservoir system to provide vacuum power to the brake servo (and other engine actuators), the pumps can fail . . .

A leak in the vacuum system in general could be a problem . . .

Some petrols use vacuum supply from the inlet manfold and a non return valve - could also be a problem . . .

The servo itself could be faulty . . .

Unlikely to be b ABS problem as the fault light would be on . . .

More info please . . .

I have full the necessary for full VAG diagnostics from the onboard systems point of view if you are stuck . . . certainly better than paying a stealer to scan the car for faults . . .

Fd

fd · Post by fd » Thu Jan 24, 2008 11:36 pm

As for being concerned - I would be . . . if you unexpectedly go back to the 'first thing in the morning' mode hen you're not expecting it you'll have a big accident . . .

$0.02

Fd

Rich H · Post by **Rich H** » Fri Jan 25, 2008 12:09 am

I don't own a car with ABS bollocks on it... But FD knows his stuff, makes sense!

thinfourth · Post by **thinfourth** » Fri Jan 25, 2008 12:58 am

quick test

With car parked and engine off push the brakes a good few times till the pedal feels solid. Now press the brakes moderatly hard and start the engine while still pushing the brakes.

Report back on what the pedal did as it should go soft almost as soon as the engine is running

Mr Momo · Post by **Mr Momo** » Sat Jan 26, 2008 7:34 pm

Thanks guys - will try in the morning

Fergus - vacuum sounds like a possible - does it mean that the vacuum has leaked away slowly overnight, so no assistance until engine is on, but the leak rate is low enough to be overcome with the engine/pumped system ?

Paul - Pedal feels solids straight away (not experienced this on other cars before), but I'll see what the response is like when I turn on the engine

fd · Post by fd » Sat Jan 26, 2008 7:55 pm

Mr Momo wrote: Fergus - vacuum sounds like a possible - does it mean that the vacuum has leaked away slowly overnight, so no assistance until engine is on, but the leak rate is low enough to be overcome with the engine/pumped system ?

Yup . . . the vacuum system should retain enough vacuum to give you some brakes if the engine fails while you are driving . . . I wouldn't expect then to guarantee brake power over night (my old Audi DIDN'T for example - solid pedal in the morning), but my new(er) A6 does, but it needs vacuum to control various things on the engine so perhaps it has a larger res . . . regardless you should have full power only seconds after startup . . .

Fd

Mr Momo · Post by **Mr Momo** » Sat Jan 26, 2008 7:59 pm

So what happened with the old Audi - replace anything ?

fd · Post by fd » Sat Jan 26, 2008 10:21 pm

No problems with the old Audi . . . as soon as you started the motor the vacuum was restored and brakes were back to normal . . .

T'was an old 2.3L straight 5 petrol . . . 1/3 million miles and I still regret selling it today . . .

That engine, for example, used a vacuum pump rather then mani vacuum for the brakes . . .

Fd

Rich H · Post by **Rich H** » Sun Jan 27, 2008 9:37 am

Paul's check will confirm brake servo operation, with the engine off the servo does nothing, with the engine on the vacuum should be there immediately so you pedal will travel more. If that does not happen then start with the brake servo pipework.