M8 Greyhound clutch problem

[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 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.

Darrin,
Do you mean left hand side of the hull? That is where the high point is on ours, just at the driver's left shoulder. I notice all the other hydraulic lines (brake and throttle) run over to the right side of the hull. Maybe a previous owner took a shortcut with the clutch tubing down the left side?

Our M8 didn't have a bleed nipple at the tubing high spot so rather than install a nipple, I got rid of the high spot by running the tubing down the hill wall from the m/c and along the floor to the driver's left. Considering this is quite fragile soft 5/16" copper tubing, I'm not sure now if that was a good idea.

Perhaps the use of 1/2" tubing was to make it more resistant to getting crushed or bent, but in that case why use copper tubing?

Malcolm

[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

[Big D]

Hi Malcolm

According to the manual, and other M8s I've seen, the clutch tubing should be routed along the right hand side (co-driver side) alongside the tubing for the brakes and throttle. The bleed point should be a 90 degree fitting just inside the engine bay on the right side as Darrin says.

Your discussion around the 5/16" line is very interesting and timely for me. I was in the process of making up the lines for the clutch using 1/2" tube. Copper nickel 1/2" tube is available in the US but not easily available here. I finally found a brake specialist here who could supply it. I made up a line to use as a template out of 3/16" tube and asked them to make me the same thing in 1/2" hydraulic tubing. I wasn't confident I'd be able to fold the 1/2" tube without any kinks so figured I'd get a specialist to do it. The other lines I did in 1/4" and 5/16" came out good but even the 3/8" was getting a bit difficult to bend without kinks.

When I spoke to the brake guys, all their 'experts' asked why the hell would you need 1/2" tubing for that. I told them that's what the manual says and that's what the M8 used so go to it and make me it please! However, they came back to me on Monday and said that while they could supply the line they were having a real battle getting fittings for a 1/2" tube to match the 1/2" UNF thread on the master cylinder.

I told them about your experiences and the business owner came out and looked at the M8. He also thought 3/8" or 5/16 " tube should do the job for the clutch. He went away and had a look at the connections he could get and the only way he could get lines and connections to fit was to have a 5/16" line with an adaptor to bring it up to 1/2" UNF for the master cylinder.

So, to save any more mucking around I told him to go for it and make me that line in 5/16". Hopefully it works for me but I figure you will probably get yours resolved before I am able to test it.