Brake pedal sinking after replacement of master cylinder

[ChrisD]

Hi all.

Recently, I replaced my brake master cylinder, on my 20v ur. I bled the system, as recommended on various specialist websites, at 6 points.
However, occasionally, the brake pedal will sink way down, in certain driving scenarios – such as coasting/overrun at lower speeds.
This is obviously unsafe and quite alarming!

So, I put it down to air in the system somewhere, and decided to take it to a VW/Audi specialist for a brake fluid service, to do a proper bleed job.

This has not cured the fault and so it requires further investigation.

Since I’ve had the car, 9 yrs now, it’s always had a weak accumulator (I have a new one but not got round to fitting it yet). But, I was not getting these braking symptoms before I changed the master cylinder?

The master cyl was purchased from GSF and is def correct for my vehicle - surely this can't be at fault?

Also, if it was the accumulator that was causing this issue, surely the brake pedal would not be sinking so far – where’s all the fluid going?

Any ideas or thoughts very welcome.

Cheers.

[Sleeper_Avant]

a lip on the brake disks can cause pedal creep as it forces the pads in on an angle

obviousley there are no leaks?

try clamping each flexi hose off (not with mole grips!) one at a time, then leave the clamps on untill all 4 wheels are clamped off and if it still creeps then the master is at fault, if it stops the creep at one corner or both rear calipers, then this is where the fault is.

there are other in depth ways but this is just schoolboy stuff that normaly eradicates any issues

[DJones]

It could be a coincidence that it has shown itself following the master cylinder change but that does sound like an accumulator issue and given that you have one its a bit of a no brainer what to do next.

Its such a simple thing to change, just make sure you press the brake peddle (20+ times), with the engine off, to ensure the pressure is released.

Darren.

[ChrisD]

Hi.
Yes - you're right - no leaks.

I can see the logic in what you are saying, but, the pedal does not sink all the time. Just under certain braking conditions. Therefore, I could do your test, and it would not sink anyway.

The more I think about it, the more I suspect a defective master cyl.

Thanks for your thoughts though.

[ChrisD]

Darren.
I will be changing the bomb anyway, as it needs doing.
However, I thought the sysmtoms of a poor/failed accum would be a hard pedal, rather than the pedal sinking?

[DJones]

Darren.
I will be changing the bomb anyway, as it needs doing.
However, I thought the sysmtoms of a poor/failed accum would be a hard pedal, rather than the pedal sinking?

It may be hard (harder than usual and you have got used to it) and the sinking is what occasionally happens with a duff one.

The point i am making is you have pointed out that a part which directly relates to pedal feel and travel is defective but you are questioning a new part.

In truth you could of changed it in the time we have been corresponding, its that simple. Hence the no brainer bit..

Hope it helps.

[DJones]

It may be hard (harder than usual and you have got used to it) and the sinking is what occasionally happens with a duff one.

The point i am making is you have pointed out that a part which directly relates to pedal feel and travel is defective but you are questioning a new part.

In truth you could of changed it in the time we have been corresponding, its that simple. Hence the no brainer bit..

Hope it helps.

Incidentally the pedal travel on a good working ur, is in my view, quite a lot due to how the system operates. Which would further underwrite my point above.

My ur has the same calipers as my 996tt but the 996tt has a much shorter pedal travel than the ur.

When my accumulator went, which in truth it had done along time before i changed it, it had similar symptoms to yours. Mine would play up following hard acceleration in first then jumping on the brakes the pedal would feel good then sink, when replaced it would always travel to the sunk position but with much better feel.

Darren.

[newsh]

It could also be the proportioning valve (I think it's called?) just below the master cylinder. When this goes bad it messes with the pedal travel, I've had it happen a couple of times over the years.

[Error404]

I have found with the accumulator if its just discharged with no pressure left in it the pedal is hard. However if the diaphragm is damaged or leaking the pedal sinks like a stone. Its a horrible feeling! I would also question the quality of anything from GSF so you might have problems from both! If you take off the accumulator and shake it if you can hear fluid inside it the diaphragm has gone.