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

On 5/20/2011, Bill Wilkman wrote:
"I noticed that with the clutch pedal depressed, the car continued to move. After pumping the clutch pedal a bit, I was able to put it in gear and drive it into the garage".

First the symptoms. One day you're sitting with your foot on the clutch with transmission in gear, and the car begins to creep forward. If you're quick you might yank it out of gear or shut the engine off before it gets away from you. Or given clear traffic conditions you might just let the clutch up and continue on your way in normal fashion, until it happens again. Alternately, one day you're sitting with your foot on the brake pedal, and the pedal slowly sinks to the floor. You do a quick double pump, and all is well with the world, until it happens again.

Both of these issues have a common cause. The problem is fluid leaking internally in the master cylinder with no external leak and no fluid loss. See diagram (click for larger image).

The diagram shows the brake master cylinder parts. The clutch master is the same except without the three pieces of the valve on the left. The part of concern here is the Master Cup in the middle. When this pressure cup leaks fluid will sneak past and return to the reservoir. The Secondary Cup on the right has no pressure applied and continues to retain fluid so it doesn't leak outside. The end result will be symptoms as described above.

The cure is to rebuild the master cylinder. that is, install new seals (packing kit). If the cylinder bores are absolutely smooth and clean you might get away with simply installing new seals, which can be done without removing the master cylinder from the car. Given enough time to do it right, you should at least hone the bores a bit to help seat the new seals. If the bores are scratched or pitted deep enough that it doesn't come clean with honing, then it needs to be sleeved (cheaper then a new master cylinder assembly).

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