FLU, no clutch disengagement, and unrelated, no air pressure

Brents347

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Hey all,

So I am having two major issues with my FLU currently.

First off, I have no clutch disengagement. The clutch works (engages the engine), so I have PTO and the motor does drive the tranny, etc. I just can’t push in the clutch and stop the vehicle. I have to kill the motor. This seemed to start with a low clutch fluid level which I believed led to air in the lines. I have bled the clutch/brakes per the military manual, in the proper order, using pressure at the reservoirs OVER AND OVER but had no success. I have heard that there is another small adjustment for the clutch, that must be within a small adjustment window but I haven’t been able to find any other information about this. What am I missing? I miss having a clutch! Help! Please!

Edit to add; At the beginning of this issue, I could pump up the clutch pedal maybe 5 or even 15 times and get enough pressure out of the system to disengage the clutch briefly enough to get the truck out of gear, but that is now gone.

And secondly. While using the FLU last fall, suddenly while in the middle of a day, I lost air pressure. Not instantly, but as I used the air off the tank it was not replaced and then the tank was empty. I have checked the cross over line. It is clean (so clean it has to be a replacement). I have gone over everything from the compressor back to the tank and with the engine running I find/hear no leaks, but the FLU is so F-ing loud I don’t think I’d hear the air leak anyway. Has anyone ever had a compressor ring break, causing the compressor to not really pump air? Or what else am I missing? With the air line disconnected BEFORE the cross-over, I put my thumb over the line with the engine running and I feel a thump-thump-thump of air as the compressor piston goes up and down but it is not a major push of air. It is easy to keep my thumb over the end of the line. Anyone have any idea how strong the air pulse coming off of the compressor should be?

I appreciate any help and suggestions. I live in the mountains so the FLU has been buried under 6-8’ of snow all winter but spring is now here and I am ready to get these issues fixed and get to work!

Brent
 
Last edited:

Speedwoble

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While you push on the clutch, have soldier B look underneath and see if the slave cylinder is moving. It should be pretty clear.Off the top of my head the slave cylinder should move about 3/4 of an inch.
As to the low air pressure, I would start disconnecting lines as far forward as possible even ride up to the outlet of the air compressor. It should have plenty of pressure coming off of there like a lawnmower engine spark plug hole. Plenty to blow your finger off of the hole.
 

Brents347

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Thanks speedwobble. I have no (or very little) movement at the slave cylinder.

as far as the air, the fact that it doesn’t have enough to blow my finger off is what is concerning me...
 

Speedwoble

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I think you should invest in a new master and slave cylinder. They are relatively cheap.

For the air compressor, disconnect the hose right at the outlet and check it. If you still have very low pressure check the read valves.
 

Brents347

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Truckee, CA.
I think you should invest in a new master and slave cylinder. They are relatively cheap.

For the air compressor, disconnect the hose right at the outlet and check it. If you still have very low pressure check the read valves.
There are reed valves on the compressor cylinder? I hadn’t seen that. I will go looking.
 

frank8003

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A suggestion:
Find some place to put shop air into the system, preferably with an air dryer or at least a separator at compressor outlet. With the Deuce it was into the right rear glad hand so then could exercise all air related equipment much easier that way.
Don't have to run engine then. This allows one to search with ears and eyes while it is generally quite and inspect the workings of things. Snoop will find any leak.
 

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Brents347

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Truckee, CA.
Thanks. I do have a fairly easy way I could charge the air system from outside, so I will do that. And I orddered some snoop. Question though. The Snoop isnt a spray bottle. Do I apply it to the outside of the air system or put it inside the lines? I could do either way.
 

Speedwoble

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Thanks. I do have a fairly easy way I could charge the air system from outside, so I will do that. And I orddered some snoop. Question though. The Snoop isnt a spray bottle. Do I apply it to the outside of the air system or put it inside the lines? I could do either way.
If you are not building air pressure, you are looking for a large leak, not a pinhole. Snoop, if it is what I think it is, is applied to the outside and bubbles when exposed to a leak. Like water and dish-soap, but better.
It sounds like you are looking for a leak between the compressor and the cross-over line. That’s not a lot of real estate to check.
 

glcaines

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I think you should invest in a new master and slave cylinder. They are relatively cheap.

For the air compressor, disconnect the hose right at the outlet and check it. If you still have very low pressure check the read valves.
As Speedwoble suggests, check the output directly at the compressor before you do anything else. If the compressor has failed, there is no need to check for leaks, etc.
 

Brents347

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Truckee, CA.
First update;

No conclusions yet, but I did change out the clutch slave cylinder. Now I need to get the systems bled.

First interesting note. The clutch/rear brake reservoir cap CLEARLY states that it takes Dot 4 brake fluid. The manual states the correct military fluid, and that fluid clearly translates to Dot 5.
I ocourse can’t remember what I put in the truck last year so I am now going to drain the entire system and refill with fresh Dot 5.

Second interesting note; without tilting the cab, a clutch slave change is a B****! I ended up going in through the cab floor plate. The floor plate CAN be completely removed and lifted up through the hole into the cab, thus giving you a approximately 12” x 12” access hole. While I have it open I will bled the brakes/clutch (the clutch slave bleeder is a bitch to get to from underneath), change the shifter bushings (maybe get high range back!) and try to find the 12mm wrench I dropped under there. It never hit the ground so it’s got to be in there somewhere!

Brent
 

Speedwoble

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First update;

No conclusions yet, but I did change out the clutch slave cylinder. Now I need to get the systems bled.

First interesting note. The clutch/rear brake reservoir cap CLEARLY states that it takes Dot 4 brake fluid. The manual states the correct military fluid, and that fluid clearly translates to Dot 5.
I ocourse can’t remember what I put in the truck last year so I am now going to drain the entire system and refill with fresh Dot 5.

Second interesting note; without tilting the cab, a clutch slave change is a B****! I ended up going in through the cab floor plate. The floor plate CAN be completely removed and lifted up through the hole into the cab, thus giving you a approximately 12” x 12” access hole. While I have it open I will bled the brakes/clutch (the clutch slave bleeder is a bitch to get to from underneath), change the shifter bushings (maybe get high range back!) and try to find the 12mm wrench I dropped under there. It never hit the ground so it’s got to be in there somewhere!

Brent
Did you replace the clutch master cylinder? The clutch master cylinder can leak internally and not function with no outward failure mode. The slave cylinder will fail with visible leaking. DOT 5 is correct to use.
 

Brents347

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Truckee, CA.
Did you replace the clutch master cylinder? The clutch master cylinder can leak internally and not function with no outward failure mode. The slave cylinder will fail with visible leaking. DOT 5 is correct to use.
Don’t make me depressed speedwoble! I did not
Replace the master, I decided To start with the slave. Don’t know for sure yet if I fixed the problem, but the old slave looked horrible and did come out in two pieces.
 

glcaines

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Since you don't know what is in the systems, I would recommend flushing both the clutch and brake systems thoroughly and replacing with DOT 5 as Speedwoble suggested. DOT 5 and DOT 3 or DOT 4 are not compatible to mix. DOT 5 is far superior. Don't use DOT 5.1 as it is totally different from DOT 5, which is silicone based. The military has started using DOT 4 instead of DOT 5 in trucks with antilock brakes due to foaming issues, so there could be either in the systems or a mixture. Everything I have uses DOT 5. If you look, you can find DOT 5 surplus in bulk much less expensive.
 

Brents347

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Truckee, CA.
Since you don't know what is in the systems, I would recommend flushing both the clutch and brake systems thoroughly and replacing with DOT 5 as Speedwoble suggested. DOT 5 and DOT 3 or DOT 4 are not compatible to mix. DOT 5 is far superior. Don't use DOT 5.1 as it is totally different from DOT 5, which is silicone based. The military has started using DOT 4 instead of DOT 5 in trucks with antilock brakes due to foaming issues, so there could be either in the systems or a mixture. Everything I have uses DOT 5. If you look, you can find DOT 5 surplus in bulk much less expensive.
Corrct and I agree. I will be flushing the entire system. Wish I had time to find surplus dot 5. I paid $120 for a gallon of dot 5!

For clarification, Dot 3 and 4 are glycol based, while Dot 5 is silicone based. The two, as you said, are NOT compatible with each other. Dot 5.1 is also Glycol based and CAN be mixed with Dot 3 and Dot 4. Dot 5.1 has the highest boiling point of any of the fluids, and that is its benefit, but bo not confuse it for Dot 5. They ARE NOT compatible!

While you can mix Dot 5.1 with Dot 3 and 4 in your brake/clutch system, you will not the get the higher boiling point advantage of the Dot 5.1. You will just be dumbing down the 5.1 to the level of the 3-4.
 

Brents347

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Truckee, CA.
An update on this.

I have a clutch again! After changing both the master cylinder and the slave cylinder and getting the system bled, the clutch is working again, so that is good. To gain access to the slave cylinder (I don’t have cab-tilt capability) I removed the center floor plate and it made things MUCH easier. While I had the plate out I opened up both shifters to check the bushings. The main shifter bushing was fine (in fact I’m sure it had been replaced) but the hi/low/reverse shift bushing was gone. Now with a new bushing it goes into high range nicely.

Unfortunately, I still have no air. I changed the pressure relief valve because I was getting no air through the old one. I didn’t want to mess with rebuilding an old unit that may fail again due to age and corrosion, so I got a new one from EI. It used to be a Bosch unit, but I replaced it with a new Wabco. The cross-over line wasn’t clogged, but had been messed with and didn’t have the correct fittings so I replaced it with a new stainless steel line (Thanks Mike!). And yet, still no air. With the engine running, when I push in on the test-port on the pressure relief valve on the side of the pressure relief valve I get the pfft, pfft, pfft, pfft of air as the compressor piston cycles so I am getting air INTO the pressure relief valve, but when I go to the drivers side of the truck I am getting no air out of the new cross-over line. Nothing. Is there a way that I am supposed to adjust the new pressure relief valve? What else could be happening to the air?

My only theory is that the pressure relief valve has a spring pressure that the incoming compressed air has to overcome in order to get through the valve and maybe my compressor isn’t putting out enough pressure (or volume?).

Thoughts on this? Thanks for any help/suggestions.

Brent
 

glcaines

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Just because you are getting the pfft - pfft sound doesn't mean that your compressor is building up air pressure. I would recommend disconnecting the crossover line you replaced. Disconnect it at the drivers side. Run the engine and place your finger of the end of the line. You should feel pressure build up which will blow your finger off the end of the line. If you have pressure there, you likely have additional corroded air lines and/or one of the small regulators near the air tanks. If there is no pressure there disconnect the pressure line coming from the compressor. Disconnect it at the pressure regulator. Run the engine and place your finger over the end of the line coming from the compressor. You should feel high pressure build up which will blow your finger off of the end of the line if the compressor is working. If you don't feel high pressure against your finger, the next step is to remove the output line from the compressor and do the same thing with your finger directly on the output port on the compressor. If you have pressure at the compressor, you likely have a leak in the line from the compressor to the regulator or the line is corroded closed. If there is no pressure build up, your compressor is faulty.

A separate suggestion is to remove all three drain valves and clean out the crud. This won't help your current problem, but should be done if you haven't already done so.
 
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