When I tried to bleed the clutch on my MGB, I would open the bleed on the slave, press the pedal down. Result: the fluid comes out, the pedal stays down, so no pumping is possible (I always detach the pedal return spring, during bleeding, as it may give the false impression that the master cylinder piston has returned, where in fact it is just the pedal that has returned) it seems like the return spring inside the master is not strong enough to push back the piston. First thought: has the master return spring failed? No, on removal it is working properly.
I had spent several hours trying to bleed the clutch without success, so I looked on the internet and was comforted to see that it is a nightmare job for everyone including professional garages. While planning my next attempt, I thought I would bleed the clutch on my Triumph GT6. I filled up the master cylinder, opened the bleed. Fluid came out, pumped the pedal a few times, checked for bubbles, tightened bleed. Job completed in about 10 minutes.
Why were the two tasks so different?
This bit of information found at: mgparts.co.nz: The lack of a return valve means the air in the pipe and slave cylinder is not isolated from the master cylinder so the suction generated by the master cylinder piston stroke, is dissipated right through the system rather than confined to the master cylinder. The amount of suction remaining is not enough to draw new fluid in.
Clearly the problem lies with the master cylinder design. Taking a close look at the master cylinders in the early MGBs, there is very little room in the pedal box, the brake master cylinder takes up most of the room. It looks like MG had to use the thinnest reservoir clutch master available, and even then, they couldn’t mount it straight (see the slanted mounting flange). On later cars with servo there is more room, but they kept with the Lockheed cylinder.
The Girling master cylinder was installed in most Triumphs, Land Rovers and the MGC. There are various bores available including 3/4inch (0.75") the same bore as the MGB. The pedal box mounting bolt holes match up perfectly, but unfortunately the centre hole is a few mm too small, but a few minutes with a hand file on the master cylinder solves this:
Just swap the push rod, circlip and the retaining washer (this needs to have a notch cut out to be fitted onto the push rod):
The MGB uses a larger diameter solid pipe, so the adaptor, (as used on Land Rovers) is required to convert 3/8 to 7/16:
I had considered replacing the whole solid pipe with smaller diameter pipe, however the slave cylinder are also the larger 7/16 union.
Gently bend the solid pipe to attach to the adaptor and master cylinder. Reconnect clevis pin etc.:
Bleed clutch in normal way (as you would do the brakes) now takes about 10 minutes instead of several days.
Land Rover Master cylinder: STC500100 & Adaptor: 139082
This Girling clutch master is original equipment on the MGC, which has a completely different pedal frame again, and this picture shows masses of clearance for the clutch - which is just as well given the height of the brake master. I'm wondering if the MGC frame positions the master cylinder mounting face forwards of both MGB positions, or is just lower: