A3 brake pedal sinks, help please.

[andy10v]

Looking for any pointers or suggestions. Brake pedal sinks to floor with the engine running after brake lines and bleeding.

2002 Audi A3 1.9tdi quattro sport

Was working ok.

New OSR brake calliper
New NSR brake hardline/flexi
New OSR brake hardline/flexi

OSR inner flexi was clamped, calliper line removed and left over weekend as wrong line supplied.
New OSR brake calliper fitted
New OSR brake line fitted
New NSR brake line fitted

Bled rear brakes with the engine off, fluid came through clear, pedal feel good.

Turned on engine, brake pedal slowly sinks to floor if you keep pressure on it.
Turn off engine, brake pedal solid.
Bled rear brakes again
Bled all brake callipers
Bled master cylinder
Fitted new bleed nipples
Bled ABS pump
Bled all brakes
Bled ABS pump
Bled all brakes
Bled all brakes/master cylinder

Pedal still slowly sinks to the floor with the engine on if you keep pressure on it.

No visible leaks for brake lines/components, no signs of swelling flexis. No fluid loss (reservoir level does not drop)

Brakes were bled NSR, OSR, NSF, OSF manually with 2 people, down, open close, up repeat. ABS pump was bled through VAG COM following instructions on screen, solid fluid out both callipers, no air.

If you disconnect the servo vacuum the brake pedal feels fine, but no assist.

Current thoughts are the master cylinder seals may have been damaged/flipped after travelling the full length of the bore whilst bleeding, if there is a step worn, or a corrosion spot in the bore that is not usually swept? With the engine off the fluid bypass is so little you cannot notice it, but with the engine on the servo forces the fluid past at a noticeable rate?

Any thoughts or experiences would be appreciated.

(I think I am just trying to avoid changing the master cylinder )

Thanks

[rapido s2]

I doubt it's a master if was ok before work started, could of damaged a seal when bleeding but unlikely.
Are all the pads moving freely in the carriers, sliders moving freely? I have had to remove rear calipers to bleed before as the nipple isn't on the top it's kind of off to one side, air can get stuck in unless nipple is right up but obviously make sure the piston can't come out when bleeding.
Could also be a collapsed brake hose?

[Error404]

Conventional wisdom would point to the master cylinder, are you sure it doesn't sink at all without the engine on?
Any change with just ignition on? I had real issues bleeding the S4 until I used a pressure blender at 2bar as per the workshop manual. The 2 man method is usually fool proof though. Maybe you are just jinxed with Audis

[Monkeymagic]

Master cylinder seals have failed during full.pedal strokes during bleeding..

[andy10v]

Thanks guys, looks like Sunday will be master cylinder day...

Should I just use the pressure bleeder in future? I keep hearing horror stories of cracked reservoirs.

Or suck through with a mityvac?

[Error404]

Mityvac won't work, BTDT. You need about 2bar on a pressure bleeder I believe this is to overcome some valves in the ESP system or something. I think I used a bit less than 2bar as I was also scared of blowing the reservoir across the garage

[andy10v]

New master cylinder fitted and its exactly the same

What pressure bleeder do you have or recommend?

[rapido s2]

Seriously try bleeding the rear brakes with the nipple at the top

[Monkeymagic]

I've seen rear lines collapse internally when clamped. Have you done the abs procedure through vcds.

[andy10v]

Seriously try bleeding the rear brakes with the nipple at the top

Just cable tie a block of wood into the calliper and hold the bleed nipple up?

[andy10v]

I've seen rear lines collapse internally when clamped. Have you done the abs procedure through vcds.

So they swell back to normal under servo pressure? Any checks or diagnosis other than replace?

Bled ABS through vag com following the on screen procedure.

Also activated the ABS pump.

[Monkeymagic]

If you are getting a good flow from each then it's unlikely. You must still have air in the system. Try bleeding with engine running.

[rapido s2]

Yea just need to gut the nipple to the highest point, as they are they are about 11 o'clock. Even just taking it off and undoing the nipple and letting gravity do its stuff should work

[simon20vq]

Does sound like air in the system. I use a gunson pressure bleeding kit tenner on ebay. Should the abs be put into a 'service' setting before bleeding the system?

[Monkeymagic]

The only.times I've ever had trouble is after changing hydraulic blocks on mk5 platform but only had to cycle abs with vague com. I personally don't like the pressure breeders.