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MG MGB Technical - Clutch failure

I have a 1976 MGB Rubber Bumper which ran in to some clutch problems. I was driving out of a parking garage and I felt the bite point on the clutch getting higher and higher as I was leaving. I was able to keep going till I hit a stop sign and was unable to shift into first gear.

For a while I was convinced that I had ruined the throwout bearing (I am teaching my fiance how to drive) and I have been putting off taking out the engine. However, after reading through some of the archives I now suspect that the master cylinder might be leaking into the slave. When I press the clutch now I am pretty sure I only get the resistance of the springs and the clutch definitely will not disengage.

Does anyone have any advice about which problem it could be? Any help would be greatly appreciated.

CJK Kirkpatrick

Casey. Yes, a hydraulic system problem will give the indications you describe--and, it comes on quickly rather than building up over a period of time as a problem with the throwout bearing normally does.

Support the car on jack stands, have your fiance depress the clutch pedal while you observe the motion of the slave cylinder push rod. My measurements show an average of .363" of movement (about 3/8") when the system is working properly. If you have less than that, you should start inspecting the hydraulic system for full and proper operation before doing anything else.

Les Bengtson

"building up over a period of time as a problem with the throwout bearing normally does"

Or comes on instantly, as mine did :o)
Paul Hunt

Paul. You always seem to have more interesting adventures than I. As I remember it, over more than 40 years, I have seen a single carbon throw out bearing break up when someone depressed the clutch pedal. As I remember the circumstances, the drive felt a sudden change as the bearing's carbon surface disintegrated, followed by a loud noise as the fixed in place metal case of the bearing come into contact with the revolving surface of the pressure plate. However, it has been so long ago that I do not remember whether I was actually present when this happened or am simply remembering the story as told by a friend.

In any event, my experience is that the sudden, total disintegration of the throw out bearing is so rare that it almost becomes an event worth noting if it ever happens. Failures of the hydraulic system, on the other hand are "dead common". Hence, that is the first place I would investigate.

Les Bengtson

Mine had been making 'funny noises' for some time, and when some 60 miles from home one day it did seem to be changing, which is always a bad sign. So I drove home in top gear all the way through towns, roundabouts, traffic lights and all, and even backed it into the garage without using the clutch. Once in pressed the pedal down twice, and on the second push there was a bang and the pedal shot to the floor - an ear on the bearing had worn through and broken off.
Paul Hunt

The first thing to check is the fluid level in the clutch reservoir. It might only be low on fluid. If it's down, top it off, pump the clutch pedal and hope it comes back. After depressing the pedal and releasing each time, wait a few seconds before depressing again to allow fluid to enter the cylinder before each "pump". It's best to "bleed" it down at the slave cylinder on the passenger side of the transmission, but topping off and pumping it up is easy to do.

If the reservoir isn't low or it doesn't pump up, the most likely cause is fluid leaking past the rubber cup in the slave cylinder. It's easy to get to for replacement. Proper procedure is to hone the slave cylinder or replace it. Wear in the slave cylinder bore will wear out the replacement cup. Dirt and rust will build up in the slave cylinder over time. It's exposed to rain and such under the car when travelling and it's the lowest point in the clutch hydraulic system where heavier particles of dirt and such will settle, wear out the rubber cup and corrode the cylinder bore itself. Thankfully, it's easy to work on.

You might have problems with the clutch cylinder above the pedal at the firewall, but it's been my experience that the slave cylinder is the first place to look.
Rick Penland

"If the reservoir isn't low or it doesn't pump up, the most likely cause is fluid leaking past the rubber cup in the slave cylinder"

I think you mean fluid is leaking past the seal in the *master*. If it leaks past the seal in the slave the level in the master drops, even if fluid isn't visible on the floor.

If the master is very low then topping-up and pumping the pedal will do absolutely nothing. Bleeding using the pedal is hard enough, and even using a continuous flow method like an EeziBleed as it's trying to force air bubbles downhill in a relatively large pipe. Much easier to reverse bleed or fill, by connecting the right-hand caliper nipple to the clutch nipple, opening both, then operating the brake pedal sloooowly. You should be able to completely fill the clutch system without reducing the brake master level too much, certainly with the plastic reservoirs, but keep it topped up anyway. If bleeding a full clutch system this way siphon or pump some fluid out first to make space and avoid an overflow.
Paul Hunt

From what you describe and the fact that you still have a full pedal but the clutch won't clear tells me that your problem is most likley to be a broken diaphram spring in the clutch pressure plate. This can only be if in fact you are still getting adequate slave cyl. travel as described by Les. I've been wrong before Willy.

This thread was discussed between 09/04/2009 and 13/04/2009

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