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MG MGB Technical - Clutch Master Cylinder

I just read Leeload's brake thread and it's is a rather timely bit of info, although indirectly related to my situation with my 73B Roadster.

A couple weeks ago I had a mechanic replace the linkage stud pin for the clutch pedal (inside the pedal box). I had the car in for a couple other things and told him if he had time to go ahead and fix the "click" in the clutch pedal. The pin had a groove worn into it that put some slop in the pedal travel, causing a click/slap against my foot.

The repair was fine and I drove the car for a few days. Then I completely lost the clutch. I looked under the car and found a puddle of fluid under the slave cylinder so took it off and did a rebuild. Clutch worked great.

That is, the clutch worked great for a few days. Then the other day when I was waiting for oncoming traffic to clear to make a left turn I had the car in 1st with the clutch pedal fully depressed. Suddenly I began to loose engine RPM and the car wanted to creep forward (my foot was of course on the brake as well).

Further analysis after getting home showed the clutch worked fine if the gear shift change was completed quickly. However, if I held the pedal down while the engine was runnning the slave cylinder would creep back to it's neutral position and I'd slowly loose pedal pressure. If the engine wasn't running the pedal would maintain pressure pretty well, although not perfecty.

So, I took out the master cylinder ( are you supposed to get to that lower rear nut !!!!!....what a project!) and did a rebuild (had the kit on hand just in case!). Note: I also replaced the short rubber hose that goes to the slave cylinder and did the pedal test BEFORE pulling the master cylinder.

That was last night and the car is ready to have the Dot3 brake fluid put back into the clutch system, which I'll do later today.

Do you think I'm on the right track here and that this should solve the problem? I could see the linkage arm on the transmission (that the slave cylinder engages by about 1/2") creeping back when the clutch was loosing pressure with the pedal held all the way down and the car was running (extra clutch spring pressure or perhaps vibration made this happen when the car was running by not as bad when it was not running?).

BH Davis

Had the same thing happen years ago in a Saab. Turned out to be a pin hole in the cup in the master cyclinder.
John H


I usually rebuild the slave and master cylinder in the car, a lot easier than removing the bloody bolts. I strongly suggest using Dot 4 brake fluid which is compatable with british hydraulics. I use Castrol LMA, can be found at Auto Barn, etc.

Good luck, Rich Boris 67 B roadster
Rich Boris


What's the difference between Dot3 and Dot4?

BH Davis

DOT4 fluid is Low Moisture Absorbtion fluid. Castrol LMA is DOT4. Cheers - Dave
David DuBois

Put in the Dot4 and it seems to be okay. Rainy day today here in the northeast so won't be able to give a real road test until tomorrow.

Oh, and I guess I'll be replacing the brake fluid completely with Dot4 after flushing and replacing the fluid with Dot3 about 3 weeks ago. Oh well.......

Thanks all.
BH Davis

This thread was discussed on 04/06/2007

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