Bora sinking brake pedal

[DIYspanner]

1.9TDI 99 reg

The brakes apply but with the engine running, the pedal sinks slowly to the floor

Can't see any apparent leaks under the car but have had to put a lot of fluid in the reservoir all of a sudden.

Will take the wheels and see if there's any caliper leaks but I'm suspecting the master cylinder.

Comments please.

 

[Burnerman]

Quite possibly but did you get some air into the system? Just a bit concerned by that low fluid level - or was that just due to it not being looked at for a while?
John

 

[DIYspanner]

ref: Just a bit concerned by that low fluid level

Me too. I've top it up and monitoring it. My sister's car and I've shown her how to check it.

I just test driven it and the brakes perform well enough.
No damp patches at any of the calipers or on any of the pipes that I can see.

If the fluid drops again soon, then perhaps its leaking back into the servo unit ?

 

[Burnerman]

Obviously if the braking system hasn't been checked for a while, as the pads wear the fluid level will drop, but not to a dangerous level.
If this is the case its highly likely that the brakes will need an overhaul, especially on the back as the pads always stick in the calipers.
The master cylinder can fail in a few ways.....the seals can wear so the fluid isn't compressed but flows back through them, also the cylinder can leak at the brake rod end, into the servo as you say. (On occasion you'll see a puff of white smoke when this happens.) Unfortunately the cylinder has to be unbolted from the servo to find that one out!
Does the pedal actually hit the floor, or just go down further than you are happy with?
Try this test.....engine off, pump the pedal a few times - it should rise to the top and become rock hard. Keeping the pressure on the pedal when you start up, the pedal should sink down a bit.
John

 

[DIYspanner]

ref: Try this test

With engine off, the peddal does pump up hard.

Keeping the pressure on and starting the engine, the pedeal does sink low after about 10 seconds but not all the way down.

 

[Burnerman]

That's the traditional servo test, so all seems well there.
What state are the brakes in.....have they been stripped out and cleaned recently? I wouldn't dive in to the master cylinder replacement just yet, but if you have a helper standing by they can check for swelling of the brake flexible pipes as you press the pedal just to rule those out.
John

 

[foxhole]

When was the brake fluid last changed?

 

[DIYspanner]

ref: When was the brake fluid last changed?

The whole system has never been drained in the 8 years my sister has had the car.

What's been done though, in recent years, is all discs and pads replaced and each corner bled through because I open the bleeder when pushing the pistons back.

Have always used good quality dot4 fluid.

 

[Burnerman]

The rear brakes in particular need to be well maintained.....when was the last time they were done?
Its really a yearly event to keep them sweet on these.
A sign of master cylinder age is that you may see streaks of black in the fluid reservoir - that's actually rubber from the seals. The brake fluid can absorb water over the years, which lowers its boiling point - which is why its replacement is recommended.
John

 

[DIYspanner]

With a bit of luck, the fluid reservoir got a bit too low and let a bit air in. Topped up, the air should rise and leave ?

Does anyone know about VCDS scan tool ABS bleed procedure ? ............... I read about it here

http://forums.tdiclub.com/showthread.php?t=366442

 

[Burnerman]

Hmmm not quite as simple as that, I'm afraid......you could however have someone pressing the brake pedal while you crack open each of the 4 outlets in turn. Any air around that area should expel.
For anything else I'd use a Gunsons Eezibleed device, that pressurises the fluid in the master cylinder and keeps it topped up at the same time, and bleed at each nipple in turn.
Don't let air get into the ABS system!
John

 

[DIYspanner]

I have a one man bleeder that I've used for years - a rubber tube with a bung in one end and a slit near the same end that acts a one way valve, allowing fluid out under pedal pressure but not allowing air to enter the tube.

From the sound you can hear the "farting" of air coming out of the system or squish of fluid hitting the side of the jar you collect the bled fluid in.

Do ABS modulators have their own nipple ?

 

[Burnerman]

I've never seen a modulator with its own bleed nipple......the device you have is good enough for a bleed when you have just replaced a wheel cylinder or whatever (indeed gravity works just as well) but the Gunsons device forces pressurised fluid through the system without the 'bouncing' affect an air bubble can cause when pumping the pedal.
So how far have you got......are you still convinced the master cylinder is faulty?
John

 

[DIYspanner]

My sister will be calling round with the bora over the next few days so will get it on the ramps and check for damp pipes and bloated flexes.

I'll bleed each corner

She did a 40 mile round trip yesterday and reckons the brakes worked fine but the pedal is still soft under pressure. She check the reservoir when she got back and the level had dropped only a little as best she could tell.

 

[Burnerman]

Do consider stripping out the rear brakes for a clean!
It's also worth, when the car is up in the air, to push the brakes on hard, tug the handbrake and release.....just to see if the wheel then spins free with no binding.
John