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MG MGB Technical - Clutch Master Cylinder

Hi everyone.

I've just purchased a 1978 MGB GT, and am really excited about it. Can't wait for it to be delivered. :)

It works fine except the seam for the clutch master cylinder has gone. The owners club has told me it would be easier to replace the whole cylinder instead of the seal alone, which would be easier, especially as my knowledge of refurbs/cars is minimal.

Has anyone got any experience of this?
Is it just a case of taking the old cylinder off and putting the new one on?
Will I have to bleed the clutch fluid from the Master to remove any air?

You help for a newbie is much appreciated!

Many thanks,

M Gifford

You can rebild it or replace it. The price of a replacement is not too terrible and it is a bolt in. You will have to bleed the cluth hydraulics in either case.
J Heisenfeldt

Thanks J

Stupid question.. do I need any specialist tools to do the job, or just the average socket set?

And can you describe the best method for a newbie like me to bleed the cylinder?

I really appreciate your help.


Fairly standard tools are all that are necessary. In my 73B North american MGB, I found the banjo fitting on the back of the cylinder is very difficult to get at. Plus, the pipe is quite large in diameter, and is hard to move/adjust its position. I swore alot when I removed it the first time many years ago, and dreaded the thought of having to replace it again. More recently when it started leaking, I started by removing the cover, then took out the guts while it was still bolted into the car. I looked in to see if there was much pitting or scoring of the bore, and found it to be in good shape. I just rebuilt it while it was still in the car, and now it works perfectly. You may want to take the guts out of it before removing the whole thing, and if the bore is in good shape, it could save alot of trouble in my experience. Then again, yours being RHD, you may have much easier access to the fitting on the back of the cylinder.

I used an Ezeebleed system, which I had good success with, check the archives for others experiences.

Congrats on the purchase of the B. If you're like most you'll be hooked!

Erick Vesterback

If you are going to purchase a new clutch slave cylinder, be aware that many, if not all, of the aftermarket clutch slave cylinders are not designed to be rebuildable like the Original Equipment Lockheed clutch slave cylinder is. First, prior to installing it, make sure that both the hose (flexible pipe) and bleed nipple are in their correct locations. Because both holes use the same thread, it is easy to get them the wrong way around. The clutch slave cylinder often comes assembled in this manner so that it will fit into the box. The bleed screw should be near the high point of the slave cylinder, with the rubber flex line attached to the lower of the two holes. Inspect both the hole in the end of the pushrod as well as the clevis pin for signs of wear. It is common for both of them to have to be replaced. The bore should be closely inspected as well. If the bore is pitted, then rebuilding the slave cylinder without sleeving the bore will work for a short time, but it will probably soon be leaking again. Next, inspect both the plain shank of the mounting bolt and the bushing of the clutch throw-out arm for wear and replace them if you find any. This will greatly reduce slop in the action of the clutch pedal and make engagement more consistent. The internal cylinder spring will take up any free play in the clutch release mechanism. The internal cylinder spring and diaphragm spring of the clutch push in opposite directions. While the diaphragm spring pushes the piston back into the slave cylinder (and the fluid back into the master cylinder), the spring inside the slave cylinder tries to push it back out again. It is the spring inside the slave cylinder pushing back out that takes up the play in the mechanical linkages to the diaphragm, and the wear in the friction and drive plates, in order to give a consistent biting point. The pedal spring pulls the pedal off of the master push-rod in order to stop it from rattling around and wearing the linkage, and to give a consistent pedal height. Do not omit the rubber boot when you reassemble the mechanism as its purpose is to protect the seal. Attach the rubber hose (flexible pipe) to metal line first, and then to the slave cylinder before bolting the slave cylinder to the bellhousing. Use a new copper washer when you attach the rubber hose (flexible pipe) to the slave cylinder.
Steve S.

Oh, yes, a word about bleeding the system after you've installed the clutch master cylinder-

Although the factory service manual describes a procedure for refilling and bleeding the clutch slave cylinder after it has been installed onto the bellhousing, it is easiest to bleed it before attaching it to the bellhousing. The clutch hydraulic system is a real pig to bleed because of the long vertical section of pipe with the U-bend at the top. The conventional bleeding technique requires that any air in the pipe must be pushed all the way down the relatively large-bore pipe before it can exit the nipple of the clutch slave cylinder. This is difficult enough with a continuous pressure bleeder connected to the clutch master cylinder, and well-nigh-on impossible when using the old fashioned technique of using the clutch pedal to pump the hydraulic fluid through the system. Failure to get all of the air out of the clutch hydraulic system will result in the clutch having a low biting point, or even failure to fully disengage. Unless you have either rebuilt the hydraulic system or previously flushed out all the old crud and old fluid with denatured alcohol, do not reverse bleed the system without flushing it out first, otherwise you will push the crud into the clutch master cylinder. Use a C clamp in order to prevent the piston of the slave cylinder from popping out, and then use either a gunson
Steve S.

This thread was discussed between 15/05/2008 and 18/05/2008

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