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MG MGB Technical - Clutch Master Cylinder

I just purchased a '74 1/2 "B" and the clutch was catching badly when I depressed and released the pedal as if the master cylinder was gauled and catching. I lifted the car on stands and listened closely to the master cylinder and slave cylinder while depressing the clutch pedal and the noise and roughness seem to come from the master cylinder so I started to dismantle. FIrst I drain the fluid from the master cylinder by bleeding at the slave. When I completed this I noticed that the pedal goes in and out quite smoothly now. Is this to be expected or am wrong to assume that the master cylinder is bad? DOn't know history on this.

Tom, when my clutch MC started to go it leaked and then just went out. The pedal just came up with the return spring. I have since replaced my Salve and Master cylinders. Both of them were made in Taiwan so I don't know how long they will last LOL. If the clutch will engauge and your B will go into gear smooth and move then there should be no problem. Just keep an eye on your Master and slave for leaks and weird feel of the clutch pedal and gear shift.

James, thanks for the advice. I have now refilled and bled the system without changing anything and the old grabbing and catching of the clutch pedal is back. Again, this seems to be in the MC. I guess I'll change out the MC but I would still like to know if anyone can explain why this grabbing and binding occurs only when there is fluid in the system. Is it a pretty sure bet that the MC is bad?

I would have thought it was more likely a lubrication problem at the clutch fork pivot inside the gearbox, or if you are lucky, at the pivot between the push rod and operating fork, which is outside against the slave cylinder.

If you undo the slave cylinder off the gearbox, you can see if the lever moves freely, and if the pushrod is stuck.

It could be the master cylinder as you say, If it has gone rusty inside, it will soon leak onto the pedal.
Martin Layton


The pushrod on the Slave cylinder moves freely. If I disconnect the Slave from the clutch lever how much movement should I expect? Should I be able to engage and dis-engage the clutch by hand?

What you suggest sounds reasonable. It may not be the lack of fluid that allows the pedal to move freely but rather the fact that without the fluid the slave doesn't have to move the clutch lever. I would like to just disconnect the Slave from the clutch lever and depress the clutch but I'm afraid that the Slave piston will just come out of it's cylinder.
Tom Gillett

Don't press the pedal with the slave detached. There ought to be a spring that moves the piston fully out. If you press the pedal you will likely blow the piston out of the slave as you say.

You also won't be able to operate the clutch by pushing the lever by hand unless you are incredibly strong, however, it will move forwards towards the slave freely. This is just free play .

If you can't feel any roughness look at changing the seals in the cylinders, this will let you look at the bores for rust, and the state of the seals. There is also a possibility that the rubber hose has collapsed internally, and is restricting fluid flow one way. (Some strange things happen to rubber flexible hoses)

If everything else checks out, the engine has to come out to look at the clutch and lubricate the pivot bolt for the arm, so it is rather a last resort thing to do, and if you can live with it, wait until there is a reason to remove the engine.

Martin Layton

This thread was discussed between 22/04/2007 and 24/04/2007

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