Brake pedal goes to floor randomly, no leaks, no air, brakes work fine otherwise

[Lozzy90]

Hi guys,

Recently purchased an Audi A4 1.9 Quattro Avant and loving the car, however I have a random problem with the braking system.
The pedal seemingly randomly goes to the floor with little/no braking effort mostly at low speed or constant braking situations (slowing braking while moving down a hill in slow traffic for example). If I release the pedal and press it again the brakes come right back. They perform spot on the rest of the time and haven't done this at high speeds (15mph+). When applying firm constant pressure to the pedal it doesn't sink when not moving with engine on or off.

I have done the following to the car since having it:
Released seized hand brake cables.
Replaced seals in rear calipers and dust covers on pistons.
Fitted new rear discs and pads - pistons wound back correctly.
Bled entire braking system and flushed with fresh DOT 4 fluid.
Replaced tandem pump (previous pump had been bodged - had a leaking vacuum pipe).

Very confident that the system has no leaks, fluid level has not moved at all since having it/bleeding it/seen no fluid on the ground. Also sure there is no air in any lines, have bled them all multiple times to ensure this.

I'm planning on replacing the master cylinder as it's the last thing I can think of to be at fault i.e faulty seals maybe?

Anyone else have any other ideas what could cause such a problem? I've never had such a strange problem before with any other car.

Thanks for looking!
Lozzy

[Rob69]

Think it is likely to be the master cylinder, only other thing I can think it could be brake flexi hoses that have had an old school hose clamp used on them. Did you do this when you repaired the calipers? It crushes the stiff hose internal wall and fluid can leak into the braiding and outer sheath. Check hoses carefully for signs of ballooning. Think it is likely the ms tho.

[Lozzy90]

Thanks for the reply, no I plugged the lines didn't clamp them. It was doing this problem before hand. It has had a flexi replaced by the previous owner in the past but other than that the lines and flexis all look good. Going to replace the MC and see if that fixes it.

[zollaf]

i would suspect the mc in the first instance. connect the new one up, but leave the connections to the abs pump slightly loose, then bleed as much air out as you can before tightening them fully. this will minimise the amount of air that goes through the abs pump, saving time and work.

[Col]

These can be a right old pig to bleed. Don't go replacing the master cylinder just yet.

Bleeding the rear in particular can be difficult and what helps is ondoing the rear caliper and holding as high as you can while bleeding.

I would persevere and bleed, bleed and bleed again but using a pressure bleeder.

[Rob69]

I did that rear caliper tippy up and twisty round thing when fitting new calipers on the old touran we had, extra air did come out the nooks and crannys.

[zollaf]

air in a caliper will mean the pedal goes to the floor all the time. one press, release, then a firm pedal, thats the only time it will be firm. every other time it will drop to the floor. its worth doing a complete fluid flush first but i suspect the mc seals have been damaged by either pressing the pedal all the way down to bleed and there is some rust in the lower part of the bore due to old dirty brake fluid thats not been changed, or the seals have picked up some muck from the fluid.

[nevo]

Don't ponce about, change the m/c,it used to be a problem on mk3 golfs,worked great 90% of the time then pow no pedal,second pump, and back again,dont forget to bleed the clutch also.

[Col]

I disagree.

I've encountered this a few times on VW's of the MK4 Golf, B5 Passat, B6 A4 era...

And both times, despite being just a humble diy'er I've solved it by re-bleeding.

The time that sticks in my mind was in the early 00's when my Dad brought his B5 Passat round after picking it up from a service at a so called VW indie who had cocked up a fluid change and flush. Exactly the same symptoms, brakes worked fine but the pedal would sink if constant pressure applied. They said it was "normal".

Suffice to say, it wasn't normal.

I solved it by careful and methodical re-bleeding using a humble gunsons easi-bleed.

Since then I've done countless work on VW brakes of that era, including cutting and making full sets of copper nickel hard lines, lot of flexi hose replacements, obviously a few times filling the system empty. And every time they have been buggers to bleed, including ABS pump bleeds with vagcom / vcds. But I've always got the razor sharp factory pedal feel back with no sinking.

It is up to the OP if he wants to splash out on a new master cylinder, just don't take back to the garage who failed to bleed the system after a simple flexi hose change.

[nevo]

When it goes, the pedal goes straight to the floor not spongy or limp,its a bottle tester,the squeezing of cheeks.