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LMTV Suspension lowering system

FLBurner

New member
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Location
Florida
Hey everyone,,

Ive been lurking awhile and its obvious the knowledge here on these trucks is vast, so hopefully you all can help me out

I work for a municipal land management department and we have 2 military surplus trucks on loan from the state surplus program to use as wildland fire trucks, one is a 1977 M35A3 i think as it has a Cat motor and auto trans

The other and the truck in question, is a 2002 LMTV, with the Cat 3126 engine. I don't know what that specifically makes this truck model wise.

The guy that built these into fire trucks has retired so the general maintenance and upkeep has fallen to me. While our county has a significant fleet department, being that these trucks are not fleet assets, the mechanics there are not allowed to work on these trucks, so any work must be done by our small crew (me) or contracted out.

I have some experience in working on my own vehicles, tractors in past jobs, operating Class B trucks and a forestry dozer, as well as some hobby level small engine repair so bear with me if I screw up heavy truck nomenclature or procedures. I will be posting separate thread to address some other issues.

Anyway, the problem at hand is that the air over hydraulic system seems to be working for the most part, but there is a significant fluid leak from the front driver side kneeling cylinder, and more recently, from the hand pump shaft.

Rebuilding the handpump seems to be relatively simple, but ideally I would like to simplify the system if possible to make it more reliable and approachable by our less mechanically inclined staff as they use the trucks in our fire operations.

Is there a way to effectively delete the kneeling system in order to eliminate failure points in the air over hydro system. I feel like it would be dumb to rebuild the kneeling cylinders when I'm obviously never going to load it on a transport plane. In my reading on here, i understand the system has some challenges like common return lines or something but hopefully one of you experts has a solution for me. I dont care if the hardware remains in place as i understand the cylinders can be a nightmare to remove but can they be capped or something. Id prefer to not have to remove the entire air over hydro system as it definitely makes the cab lifting easier and more likely to be done by staff doing inspections but if making it an entirely manual system is the answer then so be it.
 

aw113sgte

Well-known member
Steel Soldiers Supporter
630
940
93
Location
La Crosse, WI
Hey everyone,,

Ive been lurking awhile and its obvious the knowledge here on these trucks is vast, so hopefully you all can help me out

I work for a municipal land management department and we have 2 military surplus trucks on loan from the state surplus program to use as wildland fire trucks, one is a 1977 M35A3 i think as it has a Cat motor and auto trans

The other and the truck in question, is a 2002 LMTV, with the Cat 3126 engine. I don't know what that specifically makes this truck model wise.

The guy that built these into fire trucks has retired so the general maintenance and upkeep has fallen to me. While our county has a significant fleet department, being that these trucks are not fleet assets, the mechanics there are not allowed to work on these trucks, so any work must be done by our small crew (me) or contracted out.

I have some experience in working on my own vehicles, tractors in past jobs, operating Class B trucks and a forestry dozer, as well as some hobby level small engine repair so bear with me if I screw up heavy truck nomenclature or procedures. I will be posting separate thread to address some other issues.

Anyway, the problem at hand is that the air over hydraulic system seems to be working for the most part, but there is a significant fluid leak from the front driver side kneeling cylinder, and more recently, from the hand pump shaft.

Rebuilding the handpump seems to be relatively simple, but ideally I would like to simplify the system if possible to make it more reliable and approachable by our less mechanically inclined staff as they use the trucks in our fire operations.

Is there a way to effectively delete the kneeling system in order to eliminate failure points in the air over hydro system. I feel like it would be dumb to rebuild the kneeling cylinders when I'm obviously never going to load it on a transport plane. In my reading on here, i understand the system has some challenges like common return lines or something but hopefully one of you experts has a solution for me. I dont care if the hardware remains in place as i understand the cylinders can be a nightmare to remove but can they be capped or something. Id prefer to not have to remove the entire air over hydro system as it definitely makes the cab lifting easier and more likely to be done by staff doing inspections but if making it an entirely manual system is the answer then so be it.
Just cap off the two hydraulic lines going to/from the kneeling cylinders on the backside of the manifold.

Oh, and supposedly unbolting the cylinders is a compete pain. Probably not worth the effort.
 
Last edited:

FLBurner

New member
9
8
3
Location
Florida
Just cap off the two hydraulic lines going to/from the kneeling cylinders on the backside of the manifold.

Oh, and supposedly unbolting the cylinders is a compete pain. Probably not worth the effort.

The manifold being the box with the selectors for cab tilt, spare tire, etc...? Im not super familiar with hydraulic nomenclature so is there a certain cap or fitting I should be looking for from a hydraulic shop? Something NAPA would have?
 

Ronmar

Well-known member
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Location
Port angeles wa
Well if you are not selecting the kneeling system on the. valve, there should be no flow there, so that speaks to a control valve issue. Unless using the hand pump, its output checkvalve should halt a y flow back.into the hand pump, but the shaft seal still should not be leaking.

If you are allowed to modify these, I would(and did) replace it all with a hand pump and a 2way diverter to select between cab and tire crane., and a standalone air valve for the cab suspension. I show the pump used on my youtube chsnnel, username Rronmar. I have a drawing with part numbers i can get to you later showing the diverter controlling 2 systems with the single pump...
 

FLBurner

New member
9
8
3
Location
Florida
Well if you are not selecting the kneeling system on the. valve, there should be no flow there, so that speaks to a control valve issue. Unless using the hand pump, its output checkvalve should halt a y flow back.into the hand pump, but the shaft seal still should not be leaking.

If you are allowed to modify these, I would(and did) replace it all with a hand pump and a 2way diverter to select between cab and tire crane., and a standalone air valve for the cab suspension. I show the pump used on my youtube chsnnel, username Rronmar. I have a drawing with part numbers i can get to you later showing the diverter controlling 2 systems with the single pump...
There must be some kind of valve issue then since it seems monthly or so there’s a puddle of fluid under the front left kneeling cylinder and the air over hydro tank is empty.

I’ll definitely check out your videos
 

Lostchain

Well-known member
Steel Soldiers Supporter
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Location
Portland, OR
There must be some kind of valve issue then since it seems monthly or so there’s a puddle of fluid under the front left kneeling cylinder and the air over hydro tank is empty.
I had this exact same problem with my 2003. I went to NAPA and bought plugs for the hydraulic hoses and plugged them right at the cylinder. About a week after I plugged the first one, the second one failed and I plugged that side too. It took another 3 or 4 months for them to finally stop drooling out the residual fluid, lol. I have observed no odd behavior from having these plugged, and the cab tilt and spare crane still work fine.
 

FLBurner

New member
9
8
3
Location
Florida
I had this exact same problem with my 2003. I went to NAPA and bought plugs for the hydraulic hoses and plugged them right at the cylinder. About a week after I plugged the first one, the second one failed and I plugged that side too. It took another 3 or 4 months for them to finally stop drooling out the residual fluid, lol. I have observed no odd behavior from having these plugged, and the cab tilt and spare crane still work fine.
This is the exact kind of experience I was hoping to find here. Thanks!
 

Ronmar

Well-known member
3,825
7,427
113
Location
Port angeles wa
The ports on the valve and cylinders are all SAE-4. There is a SAE-4 - JIC-4 adapter in each port to adapt to the JIC-4 hose ends. You can get some SAE plugs, disconnect the hoses, remove the dapters and install plugs at the valve and cylinders...
 

FLBurner

New member
9
8
3
Location
Florida
I have a drawing with part numbers i can get to you later showing the diverter controlling 2 systems with the single pump
I watched your video and that really is a clean, elegant solution. If you’d be willing to share the drawing and parts list that would be really helpful.
 

Ronmar

Well-known member
3,825
7,427
113
Location
Port angeles wa
I watched your video and that really is a clean, elegant solution. If you’d be willing to share the drawing and parts list that would be really helpful.
Well there is a parts list in the video description/comments, but I don’t think the diverter was in there As I am doing my tire differently. Here is a drawing that shows the diverter part Number and how it is plumbed. I did not replace any hoses, and depending on where you put the pump, you should be able to re-use existing hoses. You will have to also install a manual valve to turn the cab suspension air on and off if you remove the control valve assembly…

IMG_3292.png
 

FLBurner

New member
9
8
3
Location
Florida
Well there is a parts list in the video description/comments, but I don’t think the diverter was in there As I am doing my tire differently. Here is a drawing that shows the diverter part Number and how it is plumbed. I did not replace any hoses, and depending on where you put the pump, you should be able to re-use existing hoses. You will have to also install a manual valve to turn the cab suspension air on and off if you remove the control valve assembly…

View attachment 910596
That is most excellent. Thank you for your expertise on this.
 
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