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MG MGB Technical - clutch master cylinder problem

out in the heat driving about 30 miles in traffic
the clutch pedal required more and more throw to operate. at home i took off the linkage box cover and
when i activated the clutch linkage i could see the pedal down below moving more than 2 inches. i rotated the clevis pin to see if there was wear, there was none.
had my supper, watched the news and looked at it again,
nothing changed. in desperation i pumped the clutch at least a dozen times, AND NOW THERE IS A 1/2 inch of free play. someone have an answer
rocco grillo

I assume that the fluid level hasn't changed in the master cylinder and that would mean that there were no leaks anywhere else in the system either. In that case there is only one thing to do about it and that is change the master cylinder.

Some may say that there is air in the system and that you should bleed it and that may be the case but if there is no fault in the cylinder then there is no way that air should have been drawn in beyond the main cup seals. Change the master cylinder.
Iain MacKintosh


If pumping the pedal improves things then it's almost ceretainly hydraulics related. It sounds like the master cylinder is failing with what is known as "long pedal" - basically fluid is seeping pass the inner seal so you may not see fluid loss.


Mike M

and general wisdom on the various boards is buy new M/C rather than refurb.

J.T. Bamford

I'd try re-sealing if you can get the correct seals as it's not much of a safety issue like the brakes would be. The seals do a lot of travelling and are probably just worn. Plus - not done it but I think you may be able to change them without removing the cyl. Perhaps someone knows?


Rich, I'm not entirely sure that changing the seals will solve the problem sometimes it does and it just might be worth a go.

Drain the fluid before you start.

Take off the box cover and remove the split pin and clevis. Remove the rubber boot and carefully remove the circlip whilst holding the piston rod. Then withdraw carefully. Replace all seals re assemble and bleed.

If you decide to fit a new cyl then that too is a straightforward job. See the article I posted about 2 months ago. It will be in the archive. No need to remove the pedal bracket or work from under the dash.
Iain MacKintosh

thank you all for the great advise, parts will be here tomorrow, wish me luck....IAIN tried your hyperlink without success, found your instructions....thanks
rocco grillo

Sorry Rocco, I broke the link to that mail box some time back. I have explained the reason in the SU fuel pump Mech vs Elec thread. Hope this is OK and hope you get sorted out.

Iain MacKintosh

Make sure the bore is smooth with no pitting or scores. If it has any of the above the seal will fail quickly or may not work at all. You can use a brake hone to smooth the bore is need be and clean it thoroughly afterwards.

Fortunately these are cast iron as aluminum can never be honed as it's hard anodized.
Mike MaGee

This thread was discussed between 12/07/2005 and 17/07/2005

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