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MG MGB Technical - Slave Cylinder Overhaul

I'm in the process of rebuilding the clutch master on my '67 B. Despite my best efforts, I can't seem to get the resevoir 100% clean. I've let it soak in an industrial solvent tank with solvent pumping through it. I've used compressed air and four cans of brake spray cleaner. The cleaner always comes out clean until I put 120PSI of air down there, then when I rinse again I get more brown color draining out. It's a very minor amount.

When is it "clean enough"? I suspect it needs to be hot tanked, but if this is simply residue that the old brake fluid (silicon) did not dissolve over the past 12 years, would there really be any harm in calling it good enough as is?

The system has been perfect for 12 years and did not leak when I took it apart. All rubber appears as new. I'm not sure what the brown is. Perhaps light surface rust under the "flap"?
Steve Simmons

Steve,

If it was good for the last 12 years and did not leak then, and is now so much the better for all your efforts surely you are good to go.

Cliff
Cliff Maddox

Oops, I should have titled the thread "master cylinder Overhaul". Thanks Cliff, that's what I was thinking. I'm just concerned with the stuff breaking up.

NEW QUESTION:

Now that I'm actually working on the slave, should it return under its own spring pressure? The new rubber bits are in place and everything is lubricated but it doesn't return on its own.
Steve Simmons

Steve. I am not sure what you are asking. Any springs associated with the clutch slave cylinder are inside the slave cylinder and could only push the slave cylinder piston outwards, not retract it. So the "should it return under its own spring pressure" question confuses me.

The spring inside the slave cylinder pushes the slave cylinder piston outwards, lightly, so that it remains square in the bore of the cylinder. Hydraulic fluid pushes the slave cylinder pistons outwards, against the pressure of the pushrod (which activates the clutch arm, which moves the throw out bearing forwards, which depresses the pressure plate). The internal spring, which is slightly compressed in use, is allowed to move outwards as the piston moves outwards. (Remember, we are only looking at about 3/8" of movement here.)

When the hydraulic pressure is released inside the clutch slave cylinder (foot off the clutch pedal or opening the bleed valve on the cylinder) the pressure plate forces the throwout bearing backwards, forcing the clutch fork to rotate on its pivot bolt, which causes the outer end to force the pushrod back into the slave cylinder, depressing the piston within the cylinder. Hydraulic fluid is forced out of the slave cylinder, back through the flex and hard lines, and into the master cylinder reservoir.

Hope this either answers your question, or provides us with a common understanding for further discussion.

Les
Les Bengtson

Thank you Les, that answers my question. I was trying to say that the spring is not strong enough to move the mechanism by itself. I wasn't sure if that was normal or if it meant that the rubber was too tight in the bore.

Your explanation is much appreciated.
Steve Simmons

Steve. We are conditioned to think of spring shaped items as being springs. Sometimes, they are simply an efficient mechanism for locating parts squarely on a centerline. Thus, a "weak spring" may not work well as a spring, but works quite well as a location mechanism. Glad to be able to help.

Les
Les Bengtson

In practice the spring *does* push the cylinder out of the bore, to take up any play in the mechanical linkages to the cover plate and wear in the flywheel, friction plate and cover plate. It is this feature that gives a consistent release/biting point and makes the clutch 'self-adjusting'. It is only when the clutch gets very worn and diaphragm spring pressure starts reducing, which eventually causes clutch slip at large throttle openings in 4th gear, that you might notice a high biting point when comparing it with a car with a good clutch. As well as from wear play can develop from slight end-float of the crankshaft 'shunting' the piston further back into its bore sometimes compared to others, and hence pushing extra fluid back into the master on these occasions. The next time you use the clutch the fixed throw of the master piston will have to take up this play first, before starting to release the clutch, which would reveal itself as a sudden and unexpected *low* releasing/biting point, which may well return to its expected position the next time you try.

It may well be that the spring doesn't have the force to push a dry seal down the bore, but it should do when fluid is present and with the normal vibration of engine running and movement.
Paul Hunt 2

Paul. I see it differently. Even when the bore is wet, the spring tension is very slight. Only sufficient to keep the piston square with the centerline of the slave cylinder bore. The force of the hydraulic pressure moves the piston into relationship with the pushrod and, if the piston's spring were exerting any significant force on the pushrod, the throwout bearing would be pressed against the pressure plate causing premature wear of the bearing.

With a master cylinder, the springs do serve a spring function as they are required to move the piston(s) sufficiently rearwards to allow the system access to the holes going to the reservoir. However, master cylinder pistons are designed to be stable in their bores while the slave cylinder piston is not.

Les
Les Bengtson

This thread was discussed between 10/11/2007 and 14/11/2007

MG MGB Technical index

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