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#1 S3 Brake fluid draining from higher reservour to lower reservour
Posted: Tue Jul 20, 2021 4:08 am
by robmcbee
I have a new problem with my S3 that I hope someone can help me with. My brake fluid is draining from my higher reservour RH to my lower LH reservour. On filling the lower reservour it leaks out through the sensor wires.
I have had he car for about 4 years and this has never happened before. Anyone with any ideas how to fix this?
#2 Re: S3 Brake fluid draining from higher reservour to lower reservour
Posted: Tue Jul 20, 2021 4:54 am
by abowie
You have an internal seal leak in your booster.
viewtopic.php?t=6232
#3 Re: S3 Brake fluid draining from higher reservour to lower reservour
Posted: Tue Jul 20, 2021 5:50 am
by MarekH
As temporary fix, packing the reservoir(s) with red grease and bleeding the brakes may help. You may see the red colour appear in the wrong brake circuit, which confirms the problem. The seal and/or the bore it swipes across is/are either not smooth or corroded and the seal is sticking as the shuttle moves moves back and forth. Once the bore is not smooth (likely due to rust) brake fluid can simply weep past.
Consider relocating the slave reservoir up on the firewall next to the heater box. This means there will be no head of liquid between the two reservoirs so the only time the leaking seal will have brake fluid coming past it will be when you apply/release the brakes. It doesn't cure the problem, but only a couple of drops of brake fluid make it past the seal and you'll see that the levels between the two reservoirs change only slowly over a few weeks of of driving. You can then just syringe it back from left to right (RHD car) twice a year.
In short, it's a bad design as the failure mode has not been controlled.
kind regards
Marek
EDIT:- the only other place the circuits meet is at the PDWA, if you have one, but that is more likely to be corroded solid than leaking. (You'd have to get past two seals which haven't moved in decades to get a leak from A to B here.)
#4 Re: S3 Brake fluid draining from higher reservour to lower reservour
Posted: Wed Aug 18, 2021 8:13 am
by Woolfi
I had the same problem. A rubber / plastic piston-seal of the brake-piston of the brake-cylinder, on which you press the food was damaged and the brake-liquid moved from one circuit the the second circuit. Very quick the lower 'bottle' was overfilled und the upper 'bottle' was empty. A damaged (rusty) cylinder has killed the seal.
You can by a repair kit. If the brake cylinder is damaged , it would be a good idea to renew it. I have renewed the brake cylinder and the booster.
Regrds Wolfgang Gatza