M8 Greyhound clutch problem

[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.

[Malcolm Towrie]

Darryl, i certainly agree 1/2" tubing is way overkill for the clutch line. It's a mystery why the OEM spec'd it. I suspect a massive typo.

I don't know what version of the parts manual you have, but mine shows that the original spec was lengths of 1/2" flexible hydraulic line! That's even weirder.

As someone mentioned above (maybe you), using 1/2" tubing just makes bleeding much more difficult as you won't get enough fluid velocity to carry air bubbles along the tubing.

So that's a 1/2" UNF thread on the m/c outlet. That's why I couldn't get the as-found 1/4" NPT fitting to seal!

Malcolm

[Lynn Eades]

So guys with a 1/2" UNF thread it will require a banjo fitting and a couple of copper washers.

[Big D]

Hi Lynn

Yes, that’s what I’m going to use. It seems there was no fitting or combination of fittings to adapt the 1/2” or even 3/8” tube to the 1/2” connection on the master cylinder, thus their recommendation for 5/16” tube.

[David Herbert]

I wonder if the 1/2" tube was used so that air bubbles could move along the tube by themselves to the highest point, to be removed at the bleed point by the bulkhead. They might have had the same trouble as you guys in trying to push bubbles to the bleed on the slave cylinder. It is hard to see how this system is much different to an ordinary brake system but there is obviously some reason that they went for 1/2" unless as Malcolm suggested it is actually a mistake by the designer - seems unlikely though. The larger pipe would have made any problems due to heat worse as there would be more fluid expanding by the same % so more to accomodate.
I think I would use 5/16" tube and follow the original route and include the extra bleed point as that aught to be where air would collect by itself.

David