68 mc5a air assist clutch?

[mc5a Jon]

I don't think the air assist is working on the clutch. I have to stand on it to get it to go to the floor with or with out air pressure. How do I fix this Any suggestions?

[Tony LEE]

First check that there is actually air pressure to the cylinder once the bus is aired up.

Next thing is to remove the cylinder, dismantle it and clean all interior surfaces and then lubricate the seals.

Refit once you make sure the pivot points are also moving freely.

That was enough to make a huge difference to my MC8.

May need to check the geometry. With clutch pedal up, the cylinder should be horizontal.

While you are about it, check the clutch adjustment.

[bevans6]

I've never been able to get mine to work very well either.  Couple of things to check.  The air supply comes from the small manifold on the bulkhead that also feeds (or fed in my case) the generator belt tension cylinder, and various other cylinders back in the engine bay.  make sure someone hasn't gotten creative.  The attachment to the manifold may have a shut-off valve with a tee handle - make sure it's open.  Now you have to get under and look at the cylinder itself.  It has a clevis at each end that attaches to heim joint on the rear mount and the clutch arm.  On mine, the heim joints were completely seized and the pins through the clevis were 90% seized.  I got it out, and replaced the heim joints.  I'm about to take mine out again and see if I can get a seal kit for it and replace the hose.  I am also going to use a spring tester to see if it actually works.  It's easy to calculate how much air pressure should result in how much push, and I am going to test it.  My spring tester is an electronic scale and my bridgeport, btw, nothing fancy but highly accurate!  when you reinstall it, make sure the adjusting screw is up so that the cylinder is at or even a bit  above level.  it has pressure and is pushing all the time, but the geometry is such that the push is in-line with the lever it is trying to help move until the pedal is pushed down, theen it gets a bit of angle and can help.  the push/assistance is variable - the farther down the clutch is pushed, the more the cylinder can assist.  This is balanced by the nature of the clutch spring to get harder the farther it is pushed.  If the wrong clutch spring set is in there, it will just be really hard, that's it.

When that doesn't work I am going to replace it with a bigger cylinder!  stock is 2", a 2" cylinder will give 314 lbs of push at 100 psi, while a 2.5" cylinder will give 490 lbs of push.  If that turns out to be too much, it is simple to add a regulator to set the air pressure a bit lower.

Brian

[Tenor]

On my 7, I found another valve on the frame rail that shuts off the air supply to the air assist.  I'm not sure if the 5 series did this too.  I had to get in mine and really sock the grease to the fittings to help it out.  I have found that my clutch likes to be depressed and released a few times before I first move it. 

Glenn

[Fredward]

There is a very detailed diagram of the geometry in the manual. Very important that its adjusted right because the thing that actuates the cylinder is your foot on the pedal moving the linkage. The cylinder is always pressurized waiting for the linkage to move so it can assist. I checked, lubricated, adjusted and finally determined that the clutch was shot. It was at the end of its adjustment and the cylinder didn't move enough to start assisting.

I replaced the clutch and now the assist works great. But if air pressure is down around 95 you can tell the difference from when its up around 120 FWIW. Its still a heavy clutch to operate. My wife has to grab the steering wheel and hold on to push it down and still can't fully depress it.
Fred

[bevans6]

Fred, yours sounds just like mine.  It's working, you can tell that, but it's still quite heavy.  Changing gears is fine, stop and go traffic is hard.

brian

[busshawg]

well I'm glad you guys are discussing this, I don't believe mine has been working either. Last summer there were a few times it seemed to go easier but after hearing that it is hard to clutch even when it is working mine might be fine.

[bevans6]

I was going to pull my air cylinder off on the weekend and rebuild it and test it, but we just had our biggest, baddest snow storm of the season so far, a horrible storm for Southern Ontario - I had almost 5" of snow and I had to start my snow blower! Oh the humanity...

Brian

[Fredward]

Brian,
We've had about 40" so far this winter (I think I read that somewhere) and its been here since the second week of December. No thaw. Now what is it you were going to complain about?
Fred

[mc5a Jon]

From what I'm reading, I am temped to just keep standing on the clutch pedal! I do not have any extra money to throw at it right now, so if the

clutch is worn out I will not fix it. It has air to the cylinder and I have documentation of an overhaul of engine and transmission in 97' with VERY

few miles put on since. I would like to assume they would have replaced the clutch when the overhaul was performed. It doesn't chatter or slip.

Thanks for the advise, I have a manual and I will try adjusting it. Any words of wisdom on that? That I won't get from the manual?

Jon

[bevans6]

there is a 3/4" headed bolt with a lock nut that adjusts the height of the active end of the air cylinder.  crank it up a bit to make sure it's helping as much as it can without going much past in line with the arm.  The big knob that you lift up to engage sets the free play.  They advise one full turn from stopped, but I find about 3/4 of a turn gives me acceptable free play at the pedal.  The actual pedal height is adjustable  as well, the higher the better in my experience. 

Brian