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MG Midget and Sprite Technical - Clutch hydraulics Mystery

Ok. So I've rebuild my engine completely, xcept for the clutch operating hydraulics, left that in the car.

Now, after some 300 miles my clutch pedal has no pressure and there is no travel on the slave (I think...)it doesn't disengage.

Now the strange thing is, it happens only when the engine is warm. Sometimes i could pump it up again, but that doesn't work anymore either. So, I thought it was the main cylinder failing. Took it apart, replaced cups etc. Used my easybleed to get the air out.

Went for a drive and....presto it failed again!
So, took out slave cylinder, replaced piston, cup, etc.

Well, you guess what...it still fails sometimes, when the engine is warm.
I pull over, wait a little (short time only) and then it works again.

I have no idea where to look or what to do. Any suggestions?
Alex G Matla

Do you have a flexible hose conecting to the slave, these can breakdown.

Richard.
richard boobier

Also, is there any place where the clutch piping can be getting hot, like rerouted pipes or strange exhaust systems? And you are using real brake fluid, not too old or water contaminated?

FRM
FR Millmore

Richard, I've replaced the flexible, even though it looked perfect, not clogged on the inside.

FRM, I've got a RHD, no strange routings or exotic exhaust on the wrong side of the engine :)
Alex G Matla

Another question just came up; The slave cylinder, does the piston have to be all the way back or can it be "anywhere" in the bore?
'Cause I could fit a longer pushrod, still can push back the slave half an inch orso when fitted.
Again, nothing changed there during the rebuild, same pushrod although there is another pressure group in there, a 7,5". Roller release bearing was also already installed a few years ago.
Alex G Matla

I would guess it's still one of the cylinders faulty, master or slave, or you still have air in the system. The temperature thing is probably just coincidence, ie you have obviously been using it for a while before it fails. Have you tried simulating driving by repeatedly pressing the clutch and watching the action of the slave cylinder when it is stationary and cold to see if it eventually stops working.
Cheers John
JOHN HALL & JULIE ROBERTS

John, yes I've done that, with my hand trough the hole, pressing the pedal with the other hand (ouch, its a nasty hole with sharp edges and my big hands are a snug fit when sqeezed in). And when the pedal was gone, feeling under the car to check if the fork is still in contact with the slave. It is, so its not the fork/bearing falling back.

Alex G Matla

Alex, I think you need an assistant!
Cheers John
JOHN HALL & JULIE ROBERTS

Yes, one who is willing to look under the car whilst driving :)
Alex G Matla

Alex,

When you replaced the cups, did you hone the cylinders? If not, replacing the cups may not have helped anything because they just get torn up by the old crud or can't seal against the crud. Some people claim that aluminum cylinders should never be honed. I am not one of them.

I think the slave piston does not have to be all the way back in the bore as long as it has enough travel room left to disengage the clutch, which usually does not require the full distance. If it doesn’t have enough travel it will either blow out or hit the stop clip, assuming the master is doing its job.

Of course, you could just still have air in the system.

Charley
C R Huff

I just read a post on another board about a similar problem on a B. Seems the engine ground strap was not properly connected and the current flowing through the clutch pipe was boiling the fluid. Cleaning and reconnecting the ground fixed the problem. Could be that in your case as well.
Bill Young

Charley, the cylinders looked just fine, no damage, crud or deep scratches. I've polished them with 3m blending pad.

Bill, earth strap is in place, clean (sort of) and the slave wasn't hot when I checked for movement.

I think I am going to start completely exchanging cylinders, starting with the main cylinder.
Alex G Matla

>>> ...and the current flowing through the clutch pipe was boiling the fluid. <<<

Probably not the case, if you have a plastic tube.

-:G:-
Gryf Ketcherside

Alex,

I just went through a clutch master and slave rebuild on my Saab 96. Bleeding it drove me crazy. I almost took it apart again to see if I had put it together wrong. Just before I did, I tried one more to bleed it by loosening the pipe at the master, and that fixed it in about three repetitions.

This was after I tried pump bleeding it at the slave, and after I tried pressure bleeding it, and after I tried pumping fluid in backwards from the slave to the master. It might be worth a try on yours.

Charley
C R Huff

Charley, I've just bled it again with the easy-bleed and pushed trough a lot of fluid, there was no sign of any air.

Just took it for a drive to work. Whilst driving I periodicaly felt the pedal for pressure.
On a long stretch I felt and it was ok and then a bit later it was gone! No gear changes, constant speed (its a speedcamera checked road) and still it was gone. So then I didn't touch the pedal till I came to the traffic lights and presto, the pressure was there like it had never left.

Its driving me crazy.
Alex G Matla

Alex, I remember having a similar situation a few years back, a few months after switching from DOT 5 (synthetic) to DOT 3 brake fluid.
The contamination of one fluid by the other creates a jelly, not
conducive to good hydraulic performance. Had to rebuild both cylinders
(and wheel cyls) and flush lines with alcohol. New DOT 3 fluid, and I've been good since.
Stabbing for explanations, Doug
D Ramsay

Doug, no fluid changes, just dot4 synthetic.
Alex G Matla

I had a similar thing on my midget, the clutch would be fine for about 20 miles, and then fail, I cured it by bleeding. The theory I came to was that a very small air bubble expanded with heat from the engine and gearbox after 20 miles. Not had a problem since.

Dave
Dave Barrow

But then it would be consistant when the bubble is hot, and not be there one moment and gone the next, I think.
But then again, it only happens when the engine is warm
Alex G Matla

Ok, think I've got a sound theory on whats happening.
First I found droplets of braking fluid on my pedal cover (stainless, so very good to see).
So here's what I think happens:
The cylinder bore is a little worn right before the main cup, due to driving with a foot on the clutch pedal(Yes I do that between gear changes and in anticipation of traffic, and the PO...?).
When I press the pedal the pressure before the cup rises, but because the cylinder is slightly worn there, the cup can collapse under pressure, and this pressure forces fluid back into the reservoir. As the reservoir is quite full (because of the easy-bleed used) and the fact that I've manipulated the venting hole so that it points forward, the fluid splashes onto the cover plate.
So now there is not enough fluid before the piston, and it probably sucked back the slave cylinder. The suppletion hole in the reservoir simply isn't big enough to keep up the fluid displacement.

Fact that this only happens when the engine is warm remains a mystery to me.


Alex G Matla

Alex, have you replaced the slave rod?

Matt
Tarquin

Ehhh, why should I replace the slave rod? Or do you mean if I've changed it before the trouble started, well I haven't really....
I thought at first it was too long for the 7'5" ap clutch but it wasn't, so I welded the piece wich I had sawn off back on again. So its actually the same as before the engine rebuild.
Alex G Matla

I had always known DOT 4 fluid to be Non synthetic, but a quick search showed a brand of DOT 4 Synthetic.

Still thinking...
D Ramsay

Bl**dy Hell Alex, i thought i had the exclusive on weird car failures in Holland.
Welcome in the club of mysterious car problems! :)

Arie de Best

Well, Arie, I've had my share of weird and mysterious bugs and break downs and not just with my midget. I've always owned and driven weird old things like puch mopeds, volvo amazon, MG 1300 glider, midget.
You get used to tinkering by the roadside, in fact sometimes its puts a smile on my face.
Alex G Matla

AAAARRRGGGHGHHHGH!!! Bl**dy F*ck*ng H*ll!!!

Excuse me, had to vent.

So, replaced the main clutch cylinder and guess what?
Right, engine warm, clutch gone.

I'm depressed now, also becuase an upcoming trackday is cancelled.
Alex G Matla

Cancelation of trackdays/midget-weekends and being returned on the back of the ANWB-towtruck... how recognizable all. :(

Alex, ive learnt myself to see the bright side of midgetlife:
so it problem occurse even when the mastercylinder is replaced?
Then i know its not he mastercylinder causing the problem...

What next suspicious, replacing slavecylinder?
Anything wrong with the clutcharm?
Arie de Best

ANWB would't bring me home when my carbon clutch release exploded at Assen. The two ANWB men had seen me driving on the track....

next I'm fitting a spring to the clutch fork to see whether thats the culprit.
Alex G Matla

Nah. Yesterday I took out the push rod from the slave cylinder and made it longer, about 11 mm. (7/16")
So now the piston of the slave operates in the bottom end of the cylinder. There where no signs of leakage anywhere on the slave.

It still doesn't work, pressure keeps falling away at random.

I'm starting to think it's not a hydraulic problem...
Alex G Matla

When you say it happens only with a warm engine, is that with the car stationary?
JOHN HALL & JULIE ROBERTS

It happens stationary, ie traffic lights, as well as when cruising.

With the new main cylinder at least its driveable, get pressure/clutch back with one stroke.
Alex G Matla

This thread was discussed between 09/07/2009 and 16/07/2009

MG Midget and Sprite Technical index

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