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MG MGA - Clutch problem?

Just returned from a 5 day, 1300 mile road trip in my MGA. The car ran flawlessly but on the last 40 or so miles I noticed something odd with the clutch. It was working normally then randomly it would depress with less resistance. This happened three or four times when shifting into 4th gear but when I next used the clutch it would feel normal again. I had a quick look at the master cylinder when I got home but nothing unusual to be seen there. No sign of leaks and plenty of fluid in the resevoir. Any thoughts about what might've caused this?
Andy Bounsall

After 5 days, your leg got stronger??

Sounds like it could be either master or slave - but more like the rubber plunger in the master is leaking by - torn?

Intermittent now, but I wouldn't leave it.

AJ Mail

Yes, my initial suspicion was hydraulic rather than mechanical. Any easy way to tell if the problem is at the master or slave end? I'd rather not tear the wrong end apart needlessly.
Andy Bounsall

Andy, I think that if it was the slave that was faulty, you would be losing fluid. The master cylinder has two seals. The master cup does the work but if it leaks-by, no fluid escapes because the secondary cup keeps it in the cylinder.
Lindsay Sampford

I rebuilt the master cylinder earlier this week. Out driving for a several hours today and the clutch did the same thing a couple of times. I was able to shift gears, but the pedal went to the floor with noticably less resistance than normal. I have a repair kit for the slave cylinder as well as a new flex hose so I guess I'll do those next and see if that fixes it.
Andy Bounsall

Can't hurt if you have the parts, but like has been said if the seal in the slave goes bad it would usually be leaking fluid. If you have the boot attached solidly it could be filling up without external signs of leaking, however, my best guess would be something mechanical. A hydraulic problem is not usually intermittent like that unless it is a piston sticking or a hose deteriorating.
Jeff Schultz

I'm trying to imagine what kind of mechanical problem would result in this symptom. Anybody?
Andy Bounsall

It seems strange to me that the "problem" manifests itself as improved function!

It sounds like there is a mechanical problem causing excess friction when the pedal is pressed. Something moves/re-aligns and this extra friction goes away. For example the pedal is twisting slightly on the pivot bolt, sometimes the direction of press is such that it straightens out. Worn clevis pins and holes?

It could be at either pedal end or clutch end of the system. It doesn't sound to me like a hydraulic problem, unless it is the master or slave piston being twisted. I would check all of the hinge points (pedal, pushrods, release arm) for signs of wear/sideways movement/twisting.
N McGurk

It's a hydraulic problem in the master cylinder, sometimes not moving enough fluid. Most likely a problem with the primary seal cup, which has been a persistent problem with some rebuilt kits for several years. The usual problem is that the primary seal cup is too large in diameter causing it to drag on the cylinder wall and bot return all the way. See attached picture.

It is possible that the primary cup leaks through the first part of the stroke, allowing the pedal to go part way down before it starts moving fluid. More likely the primary cup is leaking a little, and the piston is also sometimes sticking in the bore before full return. With the piston stuck at partial stroke, the pedal will have lots of free play before the piston begins to move, and there will be insufficient fluid movement.

Solution is to find a rebuild kit that has the correct size rubber seal cups.

Barney Gaylord

Barney, why would it have started doing that in the first place? Master cylinder was replaced 7-8 years ago and it worked fine until a couple of weeks ago when this problem started happening intermittently. Having replaced the seals with parts purchased last year from Moss, the problem seems to persist...intermittently.
Andy Bounsall

Because it leaks internally or sticks intermittently. I currently have a similar problem with the clutch side of my master cylinder. It works fine when cool. When the engine compartment gets hot the clutch master leaks internally causing low pedal. The clutch master piston also sticks part way in the bore so it does not return all the way This results in large pre-travel for the clutch pedal, clutch operation very close to the floor, and sometimes re-engagement of the clutch while the pedal is held down. When it cools down it works well again.

I have been struggling with the oversize primary seal cups for years (both clutch and brake master bores). It may be time to buy the TRW/Lucas master cylinder rebuild kit to get the seals in proper size.
Barney Gaylord

If I'm following, you're saying the problem likely started a couple of weeks ago because the primary seal began to intermittently leak internally. Having rebuilt the master, the piston may now be sticking intermittently resulting in a similar symptom. Is that correct?

The symptoms you describe having with your car sounds remarkably like what I'm experiencing. I haven't noticed the clutch re-engaging while the pedal is held down, but I usually only have the clutch pedal down long enough to shift and/or start moving.

Do you know a source for the TRW/Lucas master rebuild kit?
Andy Bounsall

I think you got it. On the brake side, when the primary cup leaks fluid from the working side back to the reservoir, the brake pedal will gradually sink to the floor as it is being held with moderate force. On the clutch side, the clutch will gradually re-engage as the pedal is being held down.

In either case, once some fluid has escaped from the working end of the master cylinder back into the reservoir, the piston has a greater tendency to stick half way on the return stroke (as the returning fluid pressure goes to zero.

I don't know right off hand where to get the repair kit for TRW/Lucas master cylinder. Anyone who sells the new master cylinder should supply the repair kit, or at least be able to tell you where to get it. If the repair kit was not readily available I would not buy the non-standard part.

A question closer to more people would be, does anyone know where to buy a good quality repair kit for the standard master cylinder, one with primary seal cups that are NOT too large in diameter?
Barney Gaylord

This thread was discussed between 18/05/2012 and 28/05/2012

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