The MGA With An Attitude
Master Cylinder INTERNAL LEAKAGE #2 - HT-118A

For several years I have been having problems with master cylinder rebuild kits from Moss Motors and Victoria British. I installed a new master cylinder from Moss, put on the road in late 1986, running DOT-5 fluid from day one. I had no hydraulic problems for 12 years and 129,000 miles (with regular maintenance). Then finding a couple drops of fluid on the brake shoes, in 1998 I did a bumper to bumper hydraulics rebuild. This involved flushing the system with alcohol, blowing out with air, complete disassembly and replacement of every bit of rubber in the whole system including hoses. In an attempt to settle some arguments about fluid type, I installed DOT-4 fluid (expecting it to make no difference at all). Seemed like a good idea at the time, but that's when the problems began.

Soon after the rebuild the fluid was getting black and brackish with bits of rubber included. I flushed through clean fluid at 6-month intervals, but to no avail. After 18 months the master cylinder started to leak fluid onto the heater shelf, screwing up the paint of course. Since I was going to repaint it eventually, this didn't bother me too much, but as the leak got worse it needed to be fixed. I recon the rubber piston seals in the master cylinder rebuild kit were sub-standard and deteriorating in DOT-4 fluid.

In January 2002 I hosted a club tech session to do another bumper to bumper hydraulics rebuild, again replacing every bit of rubber in the system and going back to DOT-5 fluid. It didn't help much. The fluid still gets black and brackish, just not so bad. But then the internal seals began to fail, causing the fluid to leak back to the reservoir while the pedal drops to the floor. First it was the brake side, going through a couple of rebuild kits. That seems to be resolved about 5 years ago, but then the clutch side started giving similar problems. I used kits form Moss (lasted 2 years), then V.B. (lasted 14 months), then Moss again (lasted 8 months). More recently I've been replacing only the primary pressure cup in the clutch side, which seems to work immediately but fails again in short order. A couple months ago I replaced the same piece again with the same failure immediately, no improvement.

So I went to the local NAPA store and bought some generic 7/8" diameter seal cups. These have no steps or notches around the outer edges, just plain rubber cups. They are slightly shorter, and they may be slightly stiffer. I was (and still am) somewhat concerned about the required function of allowing fluid to pass the seal cup in the forward direction in the master cylinder. This will be more important in the brake side where it needs to "pump up" in case of low pedal condition. For now I put the new cup only in the clutch side, and I did a reverse bleeding job pushing fluid from bottom up to expel air back into the master cylinder. It was an instant cure, and the clutch works perfectly now. Only time will tell I suppose.

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