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MG MGB Technical - Clutch master cylinder problem

I recently had my clutch replaced and in the process the master cylinder was also replaced. There is an funny story behind the cylinder I'm retelling before I get to the actual problem. If nothing else, it will amuse me and take some of the pain away from how I'm feeling about the car at the moment.

Funny story part.
Between my mechanic and I we can't find out where it was purchased (I know this sounds really stupid but believe me it is a mystery).
I know he put it in the car because nobody else works on my car except me. I didn't put it there because I wouldn't even know where to start. He admits he must have done it but our regular supplier also has no record of ever selling either of us this part. We then checked an alternative supplier who he sometimes uses (even though he can't remember buying the part at all, let alone from this supplier). Same story. The part has just magically appeared in my car at the same time as he replaced the clutch. Anyway he never charged me for it (his invoices are very detailed).

Now the problem part.
In short, the piston is stuck inside the master cylinder. How did it happen?

At first (since a week ago) I was experiencing minor symptoms. The clutch pedal seemed a bit loose and only had pressure when it was almost fully depressed. It was just enough for the gears to be able to change without grinding. Some days it would be worse than others and I would have to time my revs just to shift gears. Getting into first or reverse was most difficult.

On the weekend I planned to bleed to system hoping to fix it. I woke up Saturday morning and it had fixed itself. The same fairy who installed the magical mystery master cylinder must have done it.
Sunday morning it was still good. Sunday afternoon I had to grind gears just to get home.
Monday (yesterday) it was good again. In the afternoon it was still good. In fact it seemed to be as good as it has ever been.
This morning I had lost almost complete use of the clutch and I had to put it in gear before I started it then do the bunny-hop thing down my driveway with the car in gear. Once I was going I was able to sync the revs to change up and by halfway to work it was going okay with no problem.
This afternoon I decided to take it to the mechanic (where the magical mystery story unfolded) but I had to do the bunny-hop to get started and it didn't improve all the way to the mechanic's shop.

Once there he started checking everything and quickly found the cause of my problem. The piston stuck inside the master cylinder.

Is this a common fault caused by a common mistake when installing a master cylinder. Is it just a common fault when dealing with a magical mystery master cylinder?
I've ordered a brand new one from my usual supplier for the mechanic to install it tomorrow. Is there anything I should warn him about so we don't have a repeat of the same problem?

Yes, I will be making numerous copies of the invoice for THIS master cylinder in case I need to sort out a warranty repair next time.

Any help or advice will be appreciated of course.
D O'Brien

Magical mystery master cylinders can exibit any and all symptoms, at any and all times, that is what makes them magical.

Ordinary master cylinders are rather boring by comparison, and certainly shouldn't have sticking pistons.

But I'm not sure what you mean by sticking anyway. It can't stick in the released position - or at least if it did you wouldn't be able to push the pedal down. And if it is sticking *down* far enough to cause grinding, and especially total non-engagement, then the clutch would be slipping in higher gears at least if not failing to engage at all. A long pedal before you can feel any back-pressure is usually air in the system as you seem to have surmised. Ordinarily I'd have said that this can't come and go, as even when air rises to the top of the pipe it is going to remain trapped in the upside-down 'U' above the master. But then there are those amongst us who claim to bleed a clutch by just leaving it overnight and it is fine next morning (perhaps the same clutch fairy as has been visiting you).
Paul Hunt

I think your problem is the hose between the master cylinder and slave cylinder. It is probably swelled inside to the point that the passage is almost closed. This happened to me once and I had the same symptoms as you have. The clutch pedal would not come up all the way after releasing it. The problem was intermittant. Try replacing the hose before you replace the master cylinder.
J Brownell

This thread was discussed between 28/07/2009 and 07/08/2009

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