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Q & A: Clutch Hydraulics

John,


I was tempted to call about this problem, but thought that it might be one worth posting as others may have a similar problem. I have checked your site's and others' tech tips and have not seen a post similar to this issue. I have a 1970 MGB that I recently purchased in very good condition. Many parts have been replaced and it is a solid car and engine. We recently took it on a 200-mile trip with no issues and excellent performance. A few weeks later, having sat in the garage unused during that time, I started it up and let it warm up for a short jaunt. Having been an MGB owner a few decade ago, I know to always check and top off all fluids before any outing and I did so in this case, in particular the clutch and brake reservoirs. We took off, running and shifting normally. One block from our house to the first stop sign, downshifting without problem. One more block to a 90-deg turn, and I note the clutch pedal feels VERY stiff. I complete the shift and have a 1 mile straight-away to the next stop sign. On trying to downshift at this point (maybe 2-3 minutes since we left the house), the clutch pedal is like a rock - immovable. Interestingly, once I put the gearbox into neutral (with engine off), I can now shift into any gear with no noise or grinding (with engine running), but it's as if all are the same as neutral (to clarify, the car doesn't move). Also note that there were absolutely no noises, grinding, or metal-on-metal sounds at all when this happened - the pedal just seemed to lock up. We towed it back home and unfortunately it sat there for 5 months due to lack of time and money. Today I started it up without problem, and found the master cylinder was dry, so I topped it up and tried to pump the pedal. Still stuck, but noticed leakage around the braided hose fitting that attaches to the slave cylinder. While it would be nice to replace both cylinders and the hose, that's a bit expensive. I disengaged the clutch pedal arm from the master cylinder by removing the connecting pin, and I'd say the master cylinder piston should have more movement than what I'm seeing, which I'd describe as "barely moving".
Also, the reservoir cap has a very slight crack in it, but enough to leak fluid when pressing the pedal, and I'm not sure that it should be air and pressure tight but that would seem to make sense. From your experience perhaps, any
idea as to what cylinder is more likely to be the problem (ie, maybe you replace 10 of one for every 1 of the other?).
Also, both rebuild kits have very few parts in them. I'm thinking that if I have a bad or jammed piston then a complete replacement would be in order. Is it worth rebuilding these cylinders, or only if they function but perhaps leak internally thus not keeping up sufficient pressure?

Thanks for your help!

- Herb

Herb,

Regarding the clutch hydraulics:

If the slave leaks, it's bad; if it's bad, it leaks. If it's not
leaking, it's OK.

If the master cylinder leaks, it's bad; BUT, it can leak internally ... read more