Brake Master Cylinder?

[SESH]

Took my series 3 OTS out on National Drive it Day on Sunday to do a 50 mile rally around the Essex Countryside. Final destination should have been Stow Maries an old WW1 airfield/museum. Anyway, after driving about 20 miles in fairly warm conditions (26 deg C) and stop start because of a large number of cars taking part, I depress the brake pedal and to my shock it goes nearly to he floor but with just enough to stop the car at a low speed. Decided wisely against completing the rally. Checked the brake fluid, but no loss, pumped the pedal for a couple of minutes and it came back up again to the normal position. I decided that it wasn't worth the risk of carrying on and left it with my mechanic, who fortunately was only 2 miles away. I've still to hear from him, but he wasn't sure of the problem, although he suspects the master cylinder. I had recently had a large service and had the brake fluid changed and brakes bled. As the fluid level in the reservoir had not gone down I am assuming a master cylinder on the way out, maybe disturbed seals with new fluid? Am I correct in my thinking? Thanks for any advice.

Mike

[lowact]

Been there done that. Start of a trip brakes seemed fine, but then after a while when u have cause to use them, pedal is soft till almost the floor. Very, very disconcerting. No leaks, bled and re-bled, reco’d the PDWA, no change. Gave it to the top specialist brake shop to fix. They bought in a retired gentleman mechanic who was keen to play with my E, until I showed him the brakes arrangement whereupon he suddenly remembered why he needed to be retired, leaving me no choice but to fix it myself. Found corrosion pitting in the master cylinder and slave cylinders, most likely due to PO travelling only 2000 miles in 20 years and rarely changing the brake fluid.

Repairing the cylinders is possible, bore and sleeve, however it was very much cheaper for me to buy, fit new master and slave cylinders (incl. servo).
Getting the slave cylinder and servo out thru the frame is not easy, but can be done, just need to twist and turn till u find the right orientation that can be folded thru the frame. The servo has 8 mounting studs, 4 on the back that bolt to the footwell firewall, 4 on the front that are unused in an E, with thread protecting plastic covers. The new servo had slightly longer studs. Cutting the front (unused) studs back to shorter than the original made installing the slave & servo a lot easier.

[SESH]

Thanks Colin,
Going to explore the Master Cylinder first and see if any issues there and probably use a master cylinder kit to replace the seals anyway. Plus still the possibility that the brake fluid boiled! Was a fairly hot day, by UK standards (26 deg C) and was in a convoy of cars, stop start in very slow traffic for about 20 miles.
Still investigating.
Cheers
Mike

[panamacarrera]

Hi Mike

Did you come right on this problem? What did you fix/change?

My s3 has the exact same problem, and not in traffic or in hot weather.

Cheers

Nigel

[SESH]

Hi Nigel,
The master cylinder was repaired with a kit and the emergency valve replaced. Has seemed to be ok since, but am now having problems with the feel of the brakes, when hot the pedal doesn't feel right and the return is slow. I have bought a new servo, so next thing is to replace it.
Mike

[panamacarrera]

Thanks for the progress report Mike.

I had previously serviced the reaction valve with red grease but that stopped the brakes binding for just a short while.

Thinking things through, I couldn't think of anything that would cause the intermittent brake problems I was having other than water in the brake fluid, which eventually boils and creates bubbles of compressible steam and spongy brakes. I've had my car a short while and previously the car had had little use for several years so fluid will as old, brake binding will have helped the brakes get hot too.

So, I've now splashed out on a new master cylinder and braided brake hoses thoroughly drained the system and filled it with fresh brake fluid and that has solved the problem.

I tested everything out with a weekend in the Lake District which involved motorway driving, crawling traffic and a good few mountain passes with no problems at all, so problem solved.

Cheers!

Nigel

[Adamski]

Glad you fixed it. I have been very successful with seal rebuild kits on my car over the years as the bores were perfect. I have heard bad stories on replacement cylinders but maybe they have been resolved now.
Did you get your PDWL centralised after? I have changed my switch as it was broken but TLC and a knack mine works perfect. Always find bleeding with engine running is essential. I have seen others disconnected!!

[panamacarrera]

Thanks Adam
Mine is an early S3, so no fancy brake valve on my car.