M1078 LMTV Hand Pump Exploded Twice

[MUR534]

Hello all,

I have looked through every thread having to do with the FMTV hydraulic system and have not been able to find anyone who has had the problem I am experiencing.

Some background info: I was a 91B in the army, but I worked on mostly FOSOV vehicles. Worked on LMTV's a bit when in garrison, but by no means am I an expert. The AOP and Hydraulic manifold were both rebuilt in 2021. I switched over to using AW-32 hydraulic oil, as it is more readily available to me, in 2022. Over the past few years, the system has worked as it should. It is also worth mentioning that the hydraulic system has been modified to operate a stage and a roof on the back of the truck. My company uses the truck as a marketing vehicle.

The problem: Last fall, while lowering the roof, the hand pump reservoir just exploded. I got a new hand pump (This one: Replacement LMTV/MTV/FMTV Manual Cab Pump (midwestmilitaryequipment.com) ) . It worked as it should for a while until yesterday when it exploded again.

My understanding is that oil will return to the hand pump until it hits a certain amount of pressure where it will then trigger a check valve in the manifold to return the rest of the oil to the AOP.

I am assuming that the check valve is not being triggered so the oil is building up pressure in the hand pump reservoir until it eventually gives in.

Has anyone ever dealt with this issue? Am I on the right track? Any insight?

Thanks in advance.

 

[Ronmar]

I think you are on the exact right track. The return path forks. One leg goes back to the hand pump reservoir. The other feeds the AOP reservoir. The AOP leg has two checks in parallel, one pointing/flowing each way. The one flowing toward the AOP has a 5PSI cracking pressure, so it should force return fluid toward the hand pump res untill it hits 5PSI, then it allows fluid to return to the AOP... sounds like that one has plugged.

The other check is 1PSI. When you use the hand pump and its sealed reservoir pulls more than 1 PSI worth of vacume, it will allow fluid to flow from the AOP res to feed the hand pump res...

 

[MUR534]

I think you are on the exact right track. The return path forks. One leg goes back to the hand pump reservoir. The other feeds the AOP reservoir. The AOP leg has two checks in parallel, one pointing/flowing each way. The one flowing toward the AOP has a 5PSI cracking pressure, so it should force return fluid toward the hand pump res untill it hits 5PSI, then it allows fluid to return to the AOP... sounds like that one has plugged.

The other check is 1PSI. When you use the hand pump and its sealed reservoir pulls more than 1 PSI worth of vacume, it will allow fluid to flow from the AOP res to feed the hand pump res...

Thanks for the reply Ronmar. I have read a lot of your contributions in LMTV posts. You really know your stuff. I've been combing through the TM's. I can't find exactly where that valve is. Probably user error. Is it one of these vavles on the side of the manifold? Would you mind pointing me in the right direction?

Thanks again.

 

[aw113sgte]

Thanks for the reply Ronmar. I have read a lot of your contributions in LMTV posts. You really know your stuff. I've been combing through the TM's. I can't find exactly where that valve is. Probably user error. Is it one of these vavles on the side of the manifold? Would you mind pointing me in the right direction?

Thanks again.

I think that's correct, I also couldn't find definitive information on the check valves. I pulled those two parts out and it wasn't totally clear if they are the check valves.

 

[Ronmar]

Unfortunately i cannot tell you for certain, as i removed my manifold and AOP for a larger volume hand pump years ago, before I ever played with it much.

There are half a dozzen checks, depending on which valve type you have. All 3 plugs on the rearward face of the manifold are checks. One is a simple ball check with spring(allen head plug?), and i would suspect the cartridge type units in your diagram are pressure set types(have a rated opening pressure).

One style valve has 2 filters and a check on the top, a cartridge check on the forward side and a check on the bottom rear.

The style you showed, has no check on the top and one filter, none on the forward side, and a cartridge check middle back, ball check near the top(allen plug) amd one at the bottom under a fitting...

 

[MUR534]

Unfortunately i cannot tell you for certain, as i removed my manifold and AOP for a larger volume hand pump years ago, before I ever played with it much.

There are half a dozzen checks, depending on which valve type you have. All 3 plugs on the rearward face of the manifold are checks. One is a simple ball check with spring(allen head plug?), and i would suspect the cartridge type units in your diagram are pressure set types(have a rated opening pressure).

One style valve has 2 filters and a check on the top, a cartridge check on the forward side and a check on the bottom rear.

The style you showed, has no check on the top and one filter, none on the forward side, and a cartridge check middle back, ball check near the top(allen plug) amd one at the bottom under a fitting...

Ok. Thank you. Have you done a thread or a video on how you overhauled your hydraulic system? I'd be interested to see what you did.

 

[Ronmar]

Here is the first one I did on the hydraulic changes.

 

[MUR534]

Here is the first one I did on the hydraulic changes.

That pump looks beefy. Crazy how fast it moves. Faster than the AOP.

Update. I took the manifold out and inspected all the valves. There is nothing seemingly out of place. Not sure where to go from here.

 

[MUR534]

Update. I disconnected the return line from the hand pump and the AOP. Not at the same time, but I wanted to make sure that one line wasn't clogged. There was flow from both. The flow to the hand pump was much greater. Since there is flow to the AOP, I am assuming that the check valve is working, but maybe not opening up all the way. I am going to try to replace as many valves as I can, bleed the system, and see if it will start working properly.

Any other thoughts are much appreciated.

 

[Ronmar]

You should be able to block that hand pump return line with a gauge and should see no more than about 5 PSI on it while fluid is returning, if the check back to the AOP return line is opening and flowing properly...