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MG MGB Technical - Clutch Pedal - Sudden loss of pedal pressure

Me, again!

It will be a couple days before I can get back home to check out all the possibilities, so in the meantime...

What is/are the most likely suspects for a clutch pedal suddenly failing.

My 80B (23k miles)shifts up and down through gears without a sound, and the clutch has given no cause for prior concern. Even cold starts in 3rd gear don't cause slipping.

Today, I got into car, fastened seatbelt, put on hat, started engine in neutral and pushed the clutch in - all ready to go.

Immediately I noticed something was wrong as the pedal almost fell to the floor as it had so little residence. I let up on the clutch and it returned but pushing it down was effortless, no residence at all. Of course it wouldn't shift. Turning off the engine, one can shift.

So - hydraulic failure or mechanical failure. Where shall I start? I'm leaning towards hose, or slave cylinder most likely and less likely the master cylinder.


Thanks for all your assistance.
R.W Anderson

Master cylinder, slave cylinder, or the flex line between them. Mechanical failures produce noise when they happen--i.e. the thrust bearing self destructs. Look to see if the master cylinder has fluid in it. If not, check to see if the flex hose is wet or whether there is a pool of hydraulic fluid under the slave cylinder. (Also, check for obvious signs of self destruction on these two items.) If you do not see anything obvious, the first suspect is internal seals in the clutch master cylinder. Solution: rebuild the entire hydraulic system for the clutch.

Les Bengtson

Have someone operate the clutch pedal for you - check whether the clutch arm moves about 10-12 mm. If it doesn't you have a hydraulic problem. If it does you have a mechanical problem with the clutch.
Mike Ellsmore

Most likely candidate is the slave cylinder. Get under and peel back the rubber boot. If it is wet with fluid under the boot, the slave is blown. If that's OK check the other hydraulic bits. As Les said, mechanical failure is unlikely without noise occuring first.

Mike Howlett

Surely the first thing to do is check the fluid level in the master cylinder!

If that's empty check for fluid in the footwell, if there then the master secondary seal has failed. Otherwise it's probably been lost from the flex hose or slave seal.

If still filled with fluid it's probably the master pressure seal failed.

If anything mechanical from the slave cylinder push-rod to the clutch cover plate has failed, like the release bearing, two or three pumps should be enough to push the slave piston out of the end of the cylinder together with all the fluid.

Paul Hunt

The problem turned out to be the hose at the clutch servo, the master cylinder was fine, no leak but it was essentially empty.

I filled the reservoir and was able to limp the car to center of garage to jack it up and do the hose repair.

It was a pin hole leak in the 34 year old hose.

It took 2 hrs to get to and back from the nearest parts store, and then 3 hours to take out and replace and bleed the system.

All works fine, but I'm thinking there may be a touch of air still in the line as the arm at clutch only moves about 1/2" and the pedal is really only active near the floor. It shifts up and down just fine without sign or sound of a problem.

So the $15 part required 3 hours of labor and 2 hours for part road trip. Good thing I don't pay myself much.

But this is sure a wake up to replace all the brake hoses that are also 34 years old.

Thanks for prior guidance on trouble shooting the problem.
R.W Anderson

I'd also be very careful of modern made rubber products as some seem to be cr*p, especially so it seems with petrol pipe over here still

I prefer the Goodridge style hoses, no rubber involved and very reasonable prices, I've gotten them on the clutch and brakes on my midget
Nigel Atkins

"The problem turned out to be the hose at the clutch servo"

Isn't that the vacuum hose? A leak in that wouldn't cause loss of fluid, but seals in the servo do fail and that causes loss of fluid into the body of the servo and from there can get via the vacuum hose into the engine.
Paul Hunt

I was bringing granddaughter home from school when clutch pedal went to the floor in my 1980 BGT this afternoon.
I drove the 400m to my local garage where I left the car for the weekend. Master cylinder was full.
I'll report back next week.
R G Everitt

Okay, maybe I'm using the wrong term for the hydraulic unit near the clutch, that activates the pin that moves the arm for the clutch.

The hose I replaced is between the rigid clutch fluid line and the hydraulic unit for the clutch.

I do not know of a vacuum line down under the car by the clutch.
R.W Anderson

Oh, and the key to deciding mechanical vs. hydraulics for the clutch going out, is the Big Noise question. I had the Big Noise a couple years ago on my 72B - throw out bearing self destructed into 10-12 pieces. It sounded like the rear end blew up.

In both cases the car was not drivable.
R.W Anderson

like yourself I'm not over technical or mechanical so I knew what you meant (clutch slave cylinder), those that are technical and mechanical can miss the more basic levels where correct names for item aren't known or forgotten

actually just noticed you did call it slave cylinder in your original post
Nigel Atkins

RW. You note that the clutch arm moves only about one half inch and question whether you may still have air in the system. My response is "No". Years ago, I had access to four MGs, all of them being driven regularly, and measured the movement of the clutch fork/pushrod system. The average was .362" or about 3/8". If the transmission is shifting normally and you are not noticing any other problems, I suspect that your system is working fine, just as my systems have over the years.

Glad you got the problem sorted.

Les Bengtson

If your transmission is shifting normally and your only complaint is that the pedal only starts to operate near the floor, check all of the pivot points of the clutch pedal and slave cylinder for ovaling out. This wear will increase the amount of distance required to operate your clutch. In essence; lost motion. RAY
rjm RAY

I am having a not dissimilar problem.
1963 roadster - original master cylinders and clutch slave cyl and hose. Over short period I lost clutch pedal pressure.
On inspection, fluid all around the cylinder cap. Removed cylinder - bore pitted and scored. Also push rod hole elongated and pin worn.
Initial actions - new slave, pushrods, pin. Bled the hydraulics (speed bleeder). Pressure still weak and not sufficient to operate clutch.
Next action - close inspection of hose - seemed to be leaking from the hose itself and the joint with the slave. Fitted new hose and copper washer, bled system. Still insufficient pedal pressure and push rod movement (about 7mm or a bit less than 5/16"). Clutch engages when pedal is lifted about 1/4" from floor (which makes for tyre squealing starts!!) When bleeding, there are no more air bubbles.

Based on the previous comments, either I still have air in the system, or a master cylinder pressure seal failure. There is no leakage around the master cylinder
As it is a 50 yr old original cylinder (albeit the PO "probably" fitted a seal kit) should I remove and inspect the master before spending any more time bleeding?

John Minchin


Have you inspected the condition of the master push rod, clevis pin and clutch pedal pin hole for wear/elongation? There must be some wear there in a '63 if it has not been addressed since then. Might be contributing on top of the issues you have already addressed.

If the pedal hole has elongated it can be welded and redrilled to size. When mine was done there was sufficient of the original hole drilling left on the unworn side to provide a guide to an accurate centre for the drilling. We only needed to weld from the worn side back into the centre.

R Taylor

Thanks Roger
the pedal pin and clevis were all replaced so I am pretty sure its a hydraulic problem.
The car hasn't done many km since restoration and the clutch action was fine at first - the slave cylinder was original and I suspect so is the master. Maybe the PO replaced the seals but left the scored/pitted bore. The seals may now be damaged hence gradual loss of fluid past the seals.
I just want to be sure I don't miss something obvious (like still having air in the system) before I shell out $200 on a new cylinder.
John Minchin

Update on my problem -

My "loss of clutch" proved to be slave cylinder related. This has been replaced.

Then the hose leaked ... replaced.

Bleeding is proving troublesome, as observed by R W Anderson and John Minchin, and clutch engages very low. (too low)
R G Everitt

Bleed the system with the slave cylinder detached from the transmission. Install a C clamp over the push rod and the rear of the cylinder. This will force whatever remaining air that is remaining to the top of the cylinder and out the bleeder. RAY
rjm RAY

Thanks RAY, I'll have to give it a try. When I come to think of it, I think a similar method was used when we had a problem with a Triumph Toledo in 1980.
R G Everitt

"check all of the pivot points of the clutch pedal and slave cylinder for ovaling out"

Whilst wear at the pedal, master push-rod and clevis pin will cause the biting point to be lower to the floor than it should be, similar wear at the slave end will not have the same effect as the hydraulic system automatically compensates for it, and the much greater amounts of wear on the release bearing and friction plate over the life of the clutch.

Reverse bleeding is the way to go with the clutch, as otherwise you are trying to push air bubbles down a relatively large bore pipe. You could try wedging the clutch pedal down overnight, which will compress the air bubbles, hopefully dislodge them, and allow them to float to the top of the pipe by the master, then next morning when you release the pedal the reverse flow should flush them into the reservoir. Works for brakes, and the volume flowing is much less.
Paul Hunt

Problem solved.
Cracked the speed bleeder nipple open, pulled the pushrod in as far as i could about 3 times. (I didnt need a C clamp)
Air bubbles cleared after 2nd push . Tightened speed bleeder, topped up m/c and perfect clutch action resulted. This should only be necessary when fitting new or refurbished slave that is empty of fluid.

John Minchin

Has anyone tried a modified version of the MGA continuous reverse bleed system? Connect the near front wheel brake bleeder nipple to the clutch slave cylinder bleed nipple with a length of PVC hose, open both nipples and pump your brake pedal slowly - I say modified MGA system as the MGA has a common reservoir - with the B you would need to add and remove fluid from appropriate reservoirs. Oh, and another essential tool for the tool box is a squeeze pipette 10 ml capacity for sucking out fluids from these reservoirs.
Mike Ellsmore

I've been recommending that system for years, with keeping an eye on the brake reservoir of course. I've used it for filling an empty clutch system so no need to remove fluid from the clutch master, as you stop pumping as soon as there is any sign of fluid in the master, then top-up as normal. Took hardly any more time to do it than to write it, to get full clutch travel. If bleeding a full system then pump some out of the clutch slave nipple first.
Paul Hunt

With that method, I suspect you would still need to force air out of a new or refurbed slave as I described. Once the slave is free of air, the next bleed would be as described.
I still like the Speed-bleeder nipples though. Only took me a few minutes to bleed the brakes by myself.

John Minchin

In theory you should, as the bleed nipple connects to the top of the cylinder, and the flex hose is in the middle of the cylinder end, so there would appear to be the potential for having an air-bubble trapped above the flex hose port. But like rear slaves, which have the bleed below the inlet anyway, it doesn't seem to be the case in practice.

Pushing the slave cylinder all the way back should - again in theory - be all (done two or three times) that is necessary to bleed the system anyway, as air should gather at the loop of pipe by the master, and be pushed back into the reservoir as you push the piston back, but that doesn't always work. The problem is that the slave seal is designed to have fluid pressure from behind to push it towards the open end of the cylinder, which presses the edges of the seal against the cylinder walls. When you push it in manually, and let go, the spring inside the slave will be push the piston back out. Whilst this will tend to pull fluid down from the reservoir, because of the negative pressure in the fluid chamber it is just as likely to pull air in past the seal. BT, DT.
Paul Hunt

This thread was discussed between 30/06/2014 and 16/07/2014

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