M8 Greyhound clutch problem

[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?

[Paul Singleton]

Many older vehicles have a small vent hole in the master cylinder cap. I have found that if this hole is blocked with dirt or rust, then you may have problems with bleeding the system, or operation because the fluid won’t move freely from the reservoir.

[Malcolm Towrie]

Bench bleeding the master cylinder made a difference. Got rid of a stubborn air bubble at the top of the piston bore, I guess. I have 1" travel of the slave cylinder rod with a full pedal stroke so all the air is gone. So now I can get it into first gear consistently.
I still think the clutch is engaging with the pedal too close to the floor but I'll live with it.
If it goes bad again, we need to pull the clutch to see if it has a mechanical problems like those listed above.

Paul, the vent hole is clear.

Malcolm

[Lynn Eades]

Malcolm, The line pressure valve holds maybe 2 psi. This is enough to hold the piston against the rod, the rod to the lever, etc. It prevents lost motion. Usually a clutch m/cyl. doesn't have a line pressure valve, but if there's always been one there, then it's there for a reason.
If someone has fitted a return spring, that's not supposed to be there, and it pushes the piston back up the bore, then the system can't do what it is supposed to do.
I have no knowledge of the particular set up in the Greyhound, but make general suggestions to try and help.
I have no answer on the 1/2 inch copper pipe, but I suspect it is to do with heat transfer and an increased volume of fluid. Still waiting for an explanation.

[Darrin Wright]

I had this task to do for a friend in Sydney who had a M8.
Apart from bleeding the clutch master cylinder, there are 2 other bleed points, one at the slave cylinder and one mid way in the piping, right hand side of hull, it is located to the rear of the fighting compartment of just inside the engine bay. This is due to the high point in the line and that air collected in this position.
Cannot remember the size of that pipe; however, once bled clutch worked really well and got to drive the M8 around for 15 min.

[Malcolm Towrie]

Originally Posted by Lynn Eades

Malcolm, The line pressure valve holds maybe 2 psi. This is enough to hold the piston against the rod, the rod to the lever, etc. It prevents lost motion. Usually a clutch m/cyl. doesn't have a line pressure valve, but if there's always been one there, then it's there for a reason.
If someone has fitted a return spring, that's not supposed to be there, and it pushes the piston back up the bore, then the system can't do what it is supposed to do.
I have no knowledge of the particular set up in the Greyhound, but make general suggestions to try and help.
I have no answer on the 1/2 inch copper pipe, but I suspect it is to do with heat transfer and an increased volume of fluid. Still waiting for an explanation.

Lynn, you've got me wondering now if the clutch m/c should have a residual pressure valve. Unlike a brake system where the residual pressure keeps the pistons tight against the shoes and a bit of pressure in the wheel cylinders to energize the cup seals, there is a clearance required between the clutch slave cylinder piston and the rod to ensure the fork isn't preloading the release bearing. So there doesn't seem to be any point in having a residual pressure valve.

Malcolm