M8 Greyhound clutch problem

[Malcolm Towrie]

Hi, I'm having some problems getting the clutch on our Greyhound to work. Fully depressing the pedal is not disengaging the clutch enough to select first gear without pumping the pedal. I've pressure bled multiple times but no luck so far. I don't think there is air in the line but I'll try benchbleeding the master cylinder.

Anyway, my question is about a long thread on the Restoration forum on here regarding rebuilding an M8. The owner mentions that the original tubing between the clutch master cylinder and slave cylinder was 1/2" copper. The tubing on ours is 5/16" copper. I checked the parts listing and confirmed the original tubing was 1/2". I can't see why the original tubing had to be as large as 1/2" or why the existing 5/16" tubing would affect the clutch operation. But it's a big difference so I thought I'd ask.

Malcolm

[Big D]

Hi Malcolm,

That would be the restoration of my M8 you have seen. Has the clutch on yours ever worked properly?

I to was surprised at the size of the hydraulic tubing for the clutch. As you are probably aware, the clutch slave cylinder on the M8 is quite chunky, and quite a bit bigger than the slave cylinder for the throttle. The throttle uses 3/8" hydraulic tubing though so with the lines on your clutch being even smaller than that at 5/16" doesn't seem quite right. Perhaps the line size simply doesn't allow sufficient volume of fluid to be pushed through to activate the slave cylinder?

[Russell Boaler]

Dealing with fluid flow on a day to day basis I'd say that the size of the tubing is probably due to the amount of fluid that needs to be transferred. If the pipe is small you'd wind up with a pedal that is just slow to depress rather than a clutch that won't fully disengage. It would also mean a lot more pressure being needed at the master cylinder to push the fluid through the pipe (compared to the larger pipe) if depressing the clutch at normal speed. Maybe that increased pressure is causing some bypassing in the master cylinder?

[Malcolm Towrie]

Originally Posted by Russell Boaler

Dealing with fluid flow on a day to day basis I'd say that the size of the tubing is probably due to the amount of fluid that needs to be transferred. If the pipe is small you'd wind up with a pedal that is just slow to depress rather than a clutch that won't fully disengage. It would also mean a lot more pressure being needed at the master cylinder to push the fluid through the pipe (compared to the larger pipe) if depressing the clutch at normal speed. Maybe that increased pressure is causing some bypassing in the master cylinder?

I agree, Russell. The pedal is easy to depress with 5/16" tubing, which is actually quite large for automotive brake and clutch systems, so I have no idea why 1/2" tubing was thought necessary.
I'll check for bypassing of the m/c cup seal by measuring what slave cylinder rod movement I get, as described above.

Malcolm

[Malcolm Towrie]

Originally Posted by Big D

Hi Malcolm,

That would be the restoration of my M8 you have seen. Has the clutch on yours ever worked properly?

I to was surprised at the size of the hydraulic tubing for the clutch. As you are probably aware, the clutch slave cylinder on the M8 is quite chunky, and quite a bit bigger than the slave cylinder for the throttle. The throttle uses 3/8" hydraulic tubing though so with the lines on your clutch being even smaller than that at 5/16" doesn't seem quite right. Perhaps the line size simply doesn't allow sufficient volume of fluid to be pushed through to activate the slave cylinder?

Darryl, yes, it was that thread on your very nice restoration I was referring to. I drove the M8 a few years ago and my recollection is it drove ok, so I think something has deteriorated.
When I saw your reference to 1/2" tubing, I said, that can't be right. I've NEVER seen 1/2" tubing on an "automotive" braking or clutch system. But the parts list confirmed it.
I can see no advantage to using tubing that large in this application. I used a pressure bleeder applying only 15 psi to the m/c reservoir and when I cracked the bleeder on the slave cylinder, I got a FOUNTAIN of fluid coming out. So 5/16" tubing seems to be quite adequate for good flow. And it's not like the tubing causes a hard pedal from squeezing fluid through restrictive tubing. The pedal effort is very light.
I did some measurements on the m/c and slave cylinder. The m/c pushes out 20 cc per stroke. The slave cylinder needs 25 cc to go its full stroke of 1 1/4". So the slave cylinder is stroking 1" when the pedal is floored. Now I need to confirm that I am getting 1" movement of the slave cylinder rod and also if 1" movement is enough to fully disengage the clutch plates.
Malcolm

[Bruce Parker (RIP)]

Getting it to work after several pumps of the pedal suggests either a leak and some air in the system somewhere or seals letting fluid past them to me.

[Lynn Eades]

Does the master cylinder have a residual line pressure valve in the end of it?
Is there a rubber seal for it to seat on?
Is the R.L.V. functioning correctly?
Has someone added a non std. return spring to the clutch fork?
Is there lost motion at the push rod? (m/cyl.)
Are the M/cyl and slave cyl. the correct bore sizes?
It sounds like you have done a good job of getting the air out, so I doubt it is the air?

[David Herbert]

Cromwell tanks have hydraulic clutch operation with 5/16" OD tubing so I rather doubt if the tube size is a problem and it should make bleeding very much easier. I know that the original 1/2" tubing can make bleeding very difficult and silicon fluid can make it impossible (it just goes round the bubbles instead of pushing them through)

David

[Malcolm Towrie]

Originally Posted by Lynn Eades

Does the master cylinder have a residual line pressure valve in the end of it?
Is there a rubber seal for it to seat on?
Is the R.L.V. functioning correctly?
Has someone added a non std. return spring to the clutch fork?
Is there lost motion at the push rod? (m/cyl.)
Are the M/cyl and slave cyl. the correct bore sizes?
It sounds like you have done a good job of getting the air out, so I doubt it is the air?

Lynn,
Yes, the m/c has a residual pressure valve. The rubber seal looks in good condition, as does the rest of the valve. I haven't checked it actually traps pressure in the line, because, to be honest, I'm not sure how a bad valve would explain what is happening.

I installed a new slave cylinder. There is a heavy return spring on the new slave cylinder that's looks the same as the one on the original slave cylinder (which was replaced because it leaked). I don't know if the clutch fork itself has an internal return spring. Why would this be a factor?

I have minimized the lost motion on the m/c in a rather desperate attempt to get a bit more m/c piston movement to get the clutch to work but it made no difference. Same for the lost motion at the slave cylinder end.

Re bore sizes, I don't know if they are stock. The new slave cylinder certainly made no difference so I assume original and new were the same bore. The m/c bore size is 1", the slave cylinder bore size is 1.25". See my post above for volumes pumped. I suspect the smaller volume pumped by the m/c is to prevent the slave cylinder bottoming out the release bearing and overloading it.

Malcolm

[David Herbert]

Is it possible that the problem lies in the clutch itself rather than the hydraulic system? I am thinking in terms of one release lever being broken so the presure plate still drags.

David

[Richard Farrant]

Originally Posted by David Herbert

Is it possible that the problem lies in the clutch itself rather than the hydraulic system? I am thinking in terms of one release lever being broken so the presure plate still drags.

David

Or even a sticking clutch plate maybe?