Weird Brake Problem!!

Fishman

Fishman

A2OC Donor
So a couple of months back I replaced the front brake hoses along with a few bits to get the car through its MOT. The brake system bled fine and met the standard expected by the tester (I'm just adding this because it might be relevant). Other than this the car has had new rear wheel cylinders and the rear drums overhauled with the last 12months (again the braking system bled and perfermed adequately).

So I got in the car a few weeks ago and the brake pedal would slowly sink to the floor. I re-bled the system and used VCDS to operate the ABS module to chase any nasties out of that..... The pedal was no better. I suspected a bad master cylinder and asked my local garage to re-bleed the system and see if it was something simple that had been overlooked.

The garage bled the system and was very happy that they had chased all the air out. The pedal was still sinking so they diagnosed a bad master cylinder that was passing fluid around the seals (yep, I was happy with this diagnosis).

After I sourced a new master cylinder from Autodoc and sent it to the garage, they have now fitted it and bled the braking system and the fault is still present.
The braking system is not leaking fluid anywhere and the brake pedal slowly sinks all the way to the floor.

ANY SUGGESTIONS
 
Brake servo diaphragm or vacuum pipe leaking. If there's a non-return valve in the vacuum circuit (not sure about this) check that too.
Mac.
 
There is a non return valve on the servo line. As it is a diesel carefully check your tandem pump. The rear port that has the corrugated pipe is the vacuum port. The fitting is known to come loose where it sits in the tandem pump, the pipe could be loose or split. If the fitting is loose other members have cleaned the area and glued and sealed it with something like JB Weld although a new tandem pump may be needed anyway.
 
Does it do it when the engine is not running? I'll wager it doesn't. You'd think this makes no sense as all the 'engine running' is doing is boosting your assist, but it often happens this way. If it doesn't, you can rule out all vacuum type issues as surely they're merely turning you bionic, they're not changing the 'wet' side at all (assuming I'm understanding this right?).

A lot of VAG products are a bit prone to allowing the pedal to sink when the engine is running and the brakes are pressed very hard for a number of seconds. Most people say 'mine doesn't', but the reality is it never really happens: you never normally press very hard on the brakes for that long. More a light to firm pressure at most.

Logic would suggest the only reasonable causes would be leakage past the master, leakage at a caliper / slave or a leaking pipe somewhere. Nevertheless, my Passat does the same: it drives perfectly fine, stops on a sixpence (even with the car trailer on the back) but if you gun the pedal with the engine running, over a few seconds the pedal will sink. No engine, vacuum depleted and pedal is rock hard for as long as you can press it.

Passes every MOT with zero issues.
 
Lets hope during the bleeding process that the pedal was not pressed too far down and damaged the seals in the master cylinder. I always place a block of wood under the pedal to prevent it being pressed too far.
 
audifan said:
Lets hope during the bleeding process that the pedal was not pressed too far down and damaged the seals in the master cylinder. I always place a block of wood under the pedal to prevent it being pressed too far.
Click to expand...
That would be both me and them the garage fitting one and them messing that one up! It's a decent garage (supposedly)
 
Worth asking how they bled the system then
Quite common now for pressure bleed systems to be used on the reservoir
One man job and less likely to damage the master inadvertently
 
Weetank2 said:
Worth asking how they bled the system then
Quite common now for pressure bleed systems to be used on the reservoir
One man job and less likely to damage the master inadvertently
Click to expand...
I used a sealey bleeding kit and the garage use a power bleeder too
 
Im hoping the garage aren't expecting me to pay them hundreds of pounds to not fix my car! After I have retrieved it I think the way forward is to do the following:

1. Clamp the 4 Flexi brake hoses and see if that makes the issue go away (I suspect not but just because I'm being thorough). Release the clamps and see if the issue re appears.

2. Fit M12 plugs to brake master cylinder and pump the pedal. If it still sinks it's a M/C issue. If the pedal feel is good the M/C is proved to be good. Reconnect pipes and check for leaks.

3. Thoroughly bleed the brake system and function the ABS unit.

After that I'm lost!
 
I agree with everyones logic and it's the conclusion I'd come to.... I just can't see anything!
 
Seals allowing the fluid to internally leak from the master cylinder back into the reservoir. No fluid escaping.
 
Did I have a bad master cylinder and then the garage have replaced it with a new one and trashed the seals whilst fitting it?
 
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