Girling Clutch Master by Crispin Allen

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.

Firstly this implies that the master piston is returning, but secondly that the bleed nipple is not being closed on the return stroke of the pedal, which is essential when clutch bleeding. Ed.

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 canted-over clutch master was rendered necessary by the North American Mk2 dual brake master, and fitted to all cars for simplicity. Before that they were upright. Ed. MGBs with the brake servo, have more room to install a different master cylinder. As the Girling Master cylinder works well on a Triumph, why not try that?

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


March 2021:

Chris Silk is unable to completely remove the cap even though he has the same dual-circuit frame and plastic cap as Crispin. Nevertheless Chris had enough room the fill the reservoir with the cap just tilted back, and although during bleeding he inadvertently emptied the reservoir twice he still completed the bleeding in 20 minutes.

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: