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MG MGB Technical - Troubleshoot clutch hydraulic issue

I just finished reassembling my '79 B today after several months of work, just in time for Drive Your British Car week. Everything appears to work, except for the clutch hydraulics. I had a small leak last summer, so this winter I rebuilt both cylinders with new rubber. I've bled the system thoroughly, but when I push the clutch pedal, it does not engage the friction plate at all. The pedal feels very squishy. There don't seem to be any obvious leaks, though. Pumping the pedal doesn't do anything.

When I have a friend push the pedal, I can see the plunger move on the slave cylinder. How far should it be moving?

It seems to me that I've got bad seals in one of the cylinders. One of the gurus in my local club suggested just replacing the cylinders, as he's never had good luck rebuilding them. But with slave cylinders running $70 and master cylinders running $80 on Victoria British, I'd only like to replace the one that's bad. How can I troubleshoot that? Are there other possibilities besides bad seals?

Thanks!
Chris St. Pierre

Chris. I measured the movement of the pushrod on a properly working system and got about 3/8" of movement. (My actual measurements were .360" to .365" with 3/8" being .375".)

Bleeding the cluch hydraulics has always been something of a problem with it always taking a little more time than expected.

If you are getting no movement of the slave cylinder pushrod, it is a cluch master cylinder problem. If you are getting movement of the pushrod, it can be a master cylinder problem, an air in the lines problem, or a slave cylinder problem. Only you can observe what is happening, thus, only you can make a determination of what is most probable.

Les
Les Bengtson

Les--

Thanks for the quick and informative response! It looks like I'm getting about 3/8" of travel as well at the slave cylinder -- the car's not on stands, so I didn't get an exact measurement, but that's what the old eyeball-and-tape-measure method showed.

Any other thoughts on how I might troubleshoot this? It seems to me that, if the pushrod is moving on the slave cylinder, the system is working up to the slave cylinder, and it would be the slave that's malfunctioning. What's the expert opinion?
Chris St. Pierre

Chris,

If the pushrod is actually moving 3/8", then it's probably not the slave cylinder, either. When you rebuilt everything, did you change the pushrod? Sometimes the hole in the rod itself is "ovaled" and even though the rod is moving, it doesn't push the clutch fork enough. Is the fork moving a comparable amount when the slave pushrod moves?

John
John English

Chris. I believe that John is on the correct track. The slave cylinder pushrod is the least critical of the two. If you have a new clutch and throwout bearing especially. It can stand quite a bit of wear, perhaps as much as one half the diameter of the original hole and still work fine. Where problems of the slave cylinder pushrod show up the most are when the carbon throw out bearing is worn down and the pushrod is worn more than a half a diameter. Under such conditions, the pushrod may not be able to move the cluch fork sufficiently. I have done no tests on this, so cannot provide a definitive point when the slave cylinder pushrod must be replaced. I use the "enlarged by half a diameter" criterium and do not have problems.

The second pushrod is the one people seldom replace--the one in the master cylinder which attaches to the clutch pedal with a clevis pin. Most of the ones I have seen are severely worn, often more than a half diameter and, sometimes, the hole has been enlarged by an entire diameter (i.e. the front to back measurement is twice that of the top to bottom measurement).

With the slave cyinder pushrod, the piston in the slave cylinder pushes the pushrod outwards, forcing the clutch fork backwards. The clutch fork has a bronze bushing through which passes a hardened steel bolt as a pivot. Thus, when the outside end is pushed backwards, the inside end, with the carbon throwout bearing, is pushed forwards against the pressure of the springs in the pressure plate. When the clutch pedal is released, the pressure is removed from the slave cylinder and the springs in the pressure plate force the carbon bearing backwards, thus forcing the pushrod back into the slave cylinder. But, it will only force the pushrod back sufficiently that the pressure plate is not longer being acted on by the throwout bearing. Hence, this functions as a self adjusting mechanism.

The master cylinder pushrod, however, does not act in this manner. The clutch pedal, like the brake pedal, has a spring that draws it's bottom section rearwards. Thus, the effect of a worn MC pushrod is to not be fully depressed when the clutch pedal is depressed. The pushrod is fully retracted by the pedal, then, when the pedal is depressed, the pushrod does not begin to move forwards until the clevis pin hits the front of the hole. I have in my odd parts collection, a clutch MC pushrod which is worn a full diameter, about .310" forwards. The effect was to not provide sufficient pressure, in an otherwise properly operating system, to fully release the clutch.

This sounds like it could be your problem (assuming that the system is fully free of air--it can take a couple of attempts to get all of the air out) and might be worth investigation. The clutch master cylinder pushrod is available as a seperate piece from Moss and may be available from other sources.

Les
Les Bengtson

Chris, had the car been sitting throughout the winter? I have had on several occasions the friction plate stick to the flywheel from sitting. The symptoms are as if the clutch hydralics are not working. I would start the engine let it warm up, then restarted the engine in first gear and while driving hit the gas and brakes and the friction plate would break free from the flywheel. I have heard of others putting the car in gear, with a volunteer in the passenger seat, clutch down while others rock the car back and forth. Sometimes the car had not been inactive for all that long. Just a thought FWIW before rebuilding the rebuild.
Dave B.
David B

Guys--

It turned out that the flywheel was stuck to the friction plate. I must have loosened it when I was fiddling, as the next morning it was unstuck. Thanks for the help!

Chris
Chris St. Pierre

This thread was discussed between 30/05/2006 and 03/06/2006

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