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MG MGB Technical - clutch headache

This 67b project of mine is a pain....i replace new master clutch seals, new slave cyl, bleed it nicely, but this is what happens.
When you first engage the gear either 1st or reverse,the car starts to move just after you release the pedal , but if you pump it twice o more, you can still engage it but only moves when pedal is almost on top. I check gearbox oil it seems a bit thick but clean (old oil)...
This car has been standing for about 6 years...
could it be the pressure plate?

Don't do that Alfredo or you will pump the piston out of the slave cyl. You are effectively depressinf the clutch pedal before all the oil has returned to the master and you can pump out the piston. That's the reason why the clutch engages further up each time. If the gears are engaged silently don't worry about it.

You seem to have more pressing problems than this to contend with !!

All the best

Iain MacKintosh

Alfredo - Pumping the clutch is just like pumping the brakes - you move the mechanism a little bit further, which causes the different engagement locations.

Sorry to say, but you probably still have some air in the clutch system, and need to bleed it some more. Many people recommend holding the clutch pedal fully in with a stick jammed against the seat frame overnight, then bleeding normally the next morning. This tends to allow all the air bubbles to collect at the top of the clutch line, then the sudden release when you remove the stick sucks them all into the master cylinder.

Makes sense to me. Best of luck - - Alec
Alec Darnall

In another words i should bleed the system again, thats good news, thanks, i will do that this weekend.

Hi Alfredo,

Very simple to know where the problem lies. If by depressing clutch pedal at max., if travel at slave cynlinder is less than 1/2inch (12 mm) there is air in system. But if it is greater than that, you could suspect problem at clutch level ( very often graphite bearing is too thin) and no matter that travel is adequate it is not enough to do the job.


Jean Guy Catford

If Alfredo is pumping up the clutch so that engagement is higher each time then whilst air in the system may be causing the original low engagement point something else is causing the pressure to leak away only slowly and so cause the progressive rise in biting point. All the pressure should release the instant the pedal is raised, for obvious reasons, you should be able to pump away on the clutch pedal and still have the normal travel of the slave and hence the normal biting point. In theory you can't get more than a single pedal-strokes worth of fluid into the system because the master piston has to come all the way back and open the port from the reservoir before it can collect some more, which should also allow all the pressure in the line to dissipate. If it isn't, then maybe the line is blocked, like a deteriorated flex hose acting as a one-way valve.
Paul Hunt

Paul, now that you mention, it is a possibility that the section of flex has decresed due to corrosion(Oil was dirty, the seals on main and slave pump were badly rust)and the amount of fluid going thru is not enough since the pressure of raising pedal is not the same as when you pump.
I should look at that when bleed again.

Agreed that in theory you can't pump the piston out but in practice you can. When the m'c piston reverses a certain amount of fluid can pass over the cup seal which of course does not seal in this direction. It is for this reason that the main pipe is 1/4 instead of the usual 3/16 as this helps quick return of fluid to the reservoir. Is your pipe 1/4 Alfredo or has somebody replaced it with the smaller bore one?
Iain MacKintosh

Iain, ill have to check tonigh on that.

Iain, the clutch pipe is 1/4" dia., i have ordered a new flex to fit.

I think you will find a similar situation when you replace the flexible but the bottom line is not the depress the pedal quickly and in rapid succession.
Iain MacKintosh

Expect cylinder cups are tired or cylinder is scored from sitting with old, wet fluid in it (rust).

If you are lucky, a hone and new cups will restore the system (I was lucky). If not then replacements are in order.

You'll like the braided line!


I will sort it out next saturday, bleed it again, and hope for the best..alf

This thread was discussed between 10/01/2005 and 13/01/2005

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