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MG MGA - Diagnosis for clutch failure

First, all components are new as of 3500 miles ago. The symptom is that with the clutch pedal fully depressed and held down under normal operation,say at a stop light,without warning it is as if I let out the clutch easily just as I would starting out from a dead stop. This is intermittent and not just a little adrenaline producing. My layman's dignosis is that the clutch slave cylinder is failing to hold pressure and that the brake fluid is pushing past the seals of the slave causing the clutch to disengage producing forward movement. As the parts are relatively low mileage parts I am surprised. My question: is this is a sound diagnosis or could the problem lie in the master cylinder? Both components are the same age. When this has happened I have thrown the tranny into nuetral and pumped the clutch pedal. The first couple of pumps there is weak pressure but on the third stroke or so normal pressure returns and the clutch works as designed. I have now parked the 1500 until I can sort this out.
David Holmes

If your clutch fluid level is going down then you have a leak. Look for drips. If it is not going down then you have a bad master cylinder.
You can replace the clutch seal cups after honing to clean it up and most likely that will be all that is needed.
I have done it with the master in place.
S Sanders

Sandy has it right. However, I must ask why are you holding the clutch in at a stop light? The MGA throwout bearing is a solid graphite block. Continuing your practice will only wear out the bearing rather rapidly. I would suggest once you get the hydraulics corrected, you change your clutching habits. Just a helpful hint.
Chuck Schaefer

Thanks Sandy and no loss of fluid. Yes, Chuck your suggestion is what I normally do but to illustrate the problem this example worked. So, I gather from you both that the problem is in the "newish" master cylinder with under 4000 miles on the clock. Seems pretty early for this problem and I wouldn't expect Having to hone the cylinder this early. Well, so much for buying new rather than sending my original off to White Post for restoration.
David Holmes

What type of brake fluid are you using David?
Lindsay Sampford

Lindsay, my thoughts too, but I was frightened to ask the question for fear of the non-sensical onslaught!
Let battle commence!
Graham M V

Graham, I did type it quietly so as not to draw too much attention! I wasn't looking to start the si***on v mi***al debate again but was just curious.
Lindsay Sampford

Just a helpful hint, as you all are talking "squishy" clutch, which would also apply to "squishy" brakes, and the brake fluid debate almost started. Have you noticed that a lot of fluid cans look the same. Not similar, but the EXACT same - even the label is the same but labeled with brake or power steering fluid. Well, trust me on this, you don't want to put PS fluid in the master cylinder. Double check your bottle before you pour. Voice of Experience, trust me on this.
Mike Parker

David check your clutch master cylinder to see if the brake non return valve has been installed on the wrong side,it can happen and the symptoms are similar to yours.
jim mckenzie

David didn't say how many years went with that "new as of 3500 miles ago". Hydraulic cup seals decompose with age, perhaps more so them with regular use. It is not uncommon for MC seals several years old to allow fluid past the edges. The internal weeping MC has bugged me a few times in recent years. Then again, I figure anything since end of first restoration in 1986 is "recent", spanning nearly a quarter million miles. Several years ago I had a new replacement set of MC seals dissolving within months of installation, and leaking externally in less than two years. There has bee a rash of bad rubber parts during the past several years. I wouldn't be too surprised if some new master cylinders were built with bad seals.
Barney Gaylord

I use dot 5 fluid... is that enough to start another battle? I use dot 3 in my 1600 with a White Post rebuild performed almost 20 years ago and no leakage at all...*Knocking on wood* I think that Barney's theory makes the most sense to me. I'll bet that a year went by for that "new" master cylinder before I added any fluid. My best guess is the "new" master cylinder was purchased in 2006. Does that coincide with the "rash of bad rubber" period? Jim, I would think that if the brake non return valve was installed on the wrong side that I would have had this problem from day 1 which I have not. This is a recent development. Thanks for all the attention. I will try to find the time to do a rebuild and report what I find. I am wondering if it will be obvious that the seals have decomposed. Also will I need to flush all the fluid in all the lines to eliminate the rubber suspended in the fluid? I'm thinking yes, to be worry free.
David Holmes

Hi David,

It should be quite obvious if the seal has failed and you may be lucky in that the fluid might not be contaminated. See what comes out when you bleed the clutch.

You may also be able to tilt the master cylinder enough to avoid having to remove altogether. Certainly that's possible on RHD cars, although it may depend a little on the pipe routing at the back.

Good thing is you don't need to worry too much about fluid spills!
Neil McGurk

Seals aside, which I agree with, my recent experience (2000 onwards) and advice I have picked up suggests that a number of batches of poorly manufactured MCs were and still are in circulation.

ALL of my problems went away when I fitted an AP Caparo MC (they have taken over production from Lockheed). Their MC is verging on twice the price of the standard Ebay offerings, but its quality is so superior.

Neil touched on contamination. Like the others, I don't want to mention the S word (shades of Macbeth!). However, in my opinion, seal degradation is often incorrectly tagged to "S". In reality, the culprit is contamination. S does not mix with other fluids and, unless the system is TOTALLY flushed with a cleansing fluid during a changeover then problems are almost certain to follow.

The following article extract may be of interest:

"When I switched over to"S" I first attempted to simply flush of the system by adding "S" and bleeding the brakes. This sounds like it should work since the two don't mix (are immiscible), and was the method suggested by the supplier. I found, however, the following:- the old brake fluid contained a lot of water, which in contact with the "S", separated into three layers: water, "S", glycol. Ordinary bleeding procedures, designed to remove air, left water in the bottom of the wheel cylinders. A sure invitation to corrosion and failure in time. However the brakes work fine at first, since any "non-compressible" fluid or mixture will work.

I have found that the best way to switch over is with a complete system overhaul. The brake lines should be flushed with acetone which dissolves the glycol gummy residues and removes any trapped moisture. The lines should be dried by blowing with air (ethyl alcohol can also be used, it is not as flammable and won't injure paint). All flexible lines and seals should be replaced so that all rubber that has been exposed to the glycol is removed. I think that the problems some have had with "S" may arise from not doing this. It may be that the problems come from the interaction of the different swelling agents in the "S" with those used in the glycol based fluid."

Steve Gyles

I have just read an interesting article from the late 80s in which an MG Metro driver suffered severe clutch judder after fitting a new slave cylinder. Investigation showed that the piston was sticking in the cylinder and moving out in jerks upon clutch release. He put a new cylinder in but got the same result. He then checked both cylinders out with an internal micrometer and got varying readings down the bores. He got both cylinders honed and all returned to normal.

I do not know which supplier made those slave cylinders but I have my suspicions! I strongly suspect that there was poor quality control with both MCs and slave cylinders back in the 80s and 90s and this still accounts for many problems being suffered today.

Steve Gyles

This thread was discussed between 21/02/2011 and 02/03/2011

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