FMTV Hydraulic Hand Pump Leaking and Blew

[abfisher1798]

My 1088A1 just threw me a curve ball. The air hydraulic pump was bad and I’ve been relying on the hand pump thus far. Today, I swapped out the air hydraulic unit with a “good“ one. During testing, I noticed a substantial leak coming from the top of the hand pump that was not evident in the past. Everything else seemed fine so I swapped the hand pump out with a spare and ran the air hydraulic again. Cab went up fine, but made it halfway down and then stopped. Also the hand lever went full up on its own and would not budge.
I slowly turned the cab manifold switch valve back and forth and that allowed the cab to come down. Then much to my surprise….POP! The hand pump blew a seal and spewed oil, in the very same spot as the one I just removed.

Maybe the good air pump is not so good after all? Or am I looking at something else?

Thanks.

 

[NDT]

The return line tees and supplies both pump’s reservoirs. Sounds like all the fluid is being forced back into the hand pump reservoir. I would study the hydraulic schematic looking for possible culprits like stuck closed check valves.

 

[Ronmar]

Sounds like the air pump is fine. I think NDT has it backwards though. Both pump outouts are Td together in the manifold. There is a check valve on the output of both pumps so they do not back feed each other.

There are some pressure steering check valves in the manifold between the two pump return lines. They are shown right in the middle of the drawing. Basically returning fluid from a valve can go right to the hand pump which is why it is not vented and the filler plug must always be in place. Once the hand reservoir is full it will build pressure on that line which will force fluid past the 5psi check and on to the AOP reservoir. If while using the hand pump the reservoir starts to go into a vacuum, it will draw additional fluid thru the 1psi check from the AOP reservoir.

Handle going up indicates the output check in the hand pump isn’t working. OR…

when the cab stopped did it stop suddenly or did it slow to a stop?

I would do as NDT suggested get out the schematic and and hand over hand the lines between manifold and both pumps. An AOP connected backwards might cause something like this as it would force fluid thru the handpump return, thru the pump chamber to its outlet(raising handle) and since fluid would be returning from the rod end of the cylinder, pressure would tend to build up in the hand pump. Same with lowering pressure would build in the hand pump till the cab stopped. Leakage as the valve Ports overlapped would allow it to continue lowering as the valve was cycled…

the quick check for this I guess would be to disconnect the lines on the return and output port on the hand pump and pump the AOP. You should only get fluid out of the disconnected output line at the hand pump. If you get fluid out the line that was
Connected to the handpump return, then the AOP is probably connected backwards…