MG Car Repair: 78 MGB Clutch, clutch master cylinder, clutch plate

Question
Hi Howard,

Your help thus far has been invaluable in the rebuild of our 78 B.

Here is another problem we just encountered. The car was modified before we got it and we now believe it has a 73/74? engine installed with twin SU's. It ran reasonably well last summer except the clutch was jerky and the synchro on third didn't work making shifts difficult.

We pulled the engine this spring and installed a gearbox from a 79 along with a new clutch plate.

Now clutch doesn't work at all, totally spongy,after repeated bleeding exercises.I suspect a bad master cylinder, my son a bad slave. What would you suspect? Lastly what would the consequences be if we had installed the clutch plate backwards?

Thanks

Fergus

Answer
Hi Fergus,
It would be difficult to install the disk backwards as the hub would hit the flywheel and bolting it up would destroy the disk and make the clutch not release at all. It would be obvious that something is wrong when you try to bolt the pressure plate up that something is wrong

The clutch master or slave cylinder can be at fault but there is a way to test the master if you can locate an assortment of brake bleeder valves at an auto parts store. Check the slave cylinder bleeder valve to see if it is the correct threads. The test is a 100% sure test if you can locate a brake bleeder that is the same threads as the line coming out of the clutch master cylinder and install the bleeder in place of the line. You just then use the bleeder valve to bleed the master cylinder and you should get a rock hard pedal that will not depress at all. If the pedal goes down slowly as you apply light pressure, the master is bad even if it does not go down if you pump it quick and hard. The slave can only leak fluid and won't fail in any other way. It is nothing but a single seal and piston. It is more difficult to get air out of a clutch system than out of the brakes because the clutch line is large and thus an air bubble will keep migrating up rather than down to the bleeder valve.

A bleeding method that works sometimes is to fill the reservoir and leave the filler cap loose and open the bleeder valve just a little so you get a drip and put a pan or whatever under the slave and just let it sit and drip. Keep an eye on the reservoir and when it is down half way, close the bleeder valve and fill the reservoir and pump the pedal in very slow long pumps. Slower on the release than on the push down.

Let me know,
Howard