M8 Greyhound clutch problem
 

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

M8
 

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?

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?

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.

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?

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

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

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

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

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?

M/C vent
 

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.

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

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.

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

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

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.

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

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

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.

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