Cab Lift Hydraulic Problem - New LMTV - Advice Needed

[Pos3idon]

Long time lurker, first time poster. I recently acquired a 1994 Stewart and Stevenson M1081. I just went to lift the cab using the hand pump to inspect the engine, the cab lifted about 3/4 of the way (while leaking a modest amount of hydraulic fluid). Once it was about 3/4 of the way up it felt like there was not enough hydraulic fluid in the system to go further so I did my checks and went to bring it back down. At first the hand pump built pressure but the cab would not move at all. Then the pump would not build pressure. The cab is still about 3/4 of the way up.

Any advice on how to get it back down? I added more fluid to the reservoir with no effect.

 

[Ronmar]

if it is only 3/4 up, As soon as you shifted the cab control to lower the cab should have come down via gravity...

1. Where was it leaking?
2. Where did you add fluid?

 

[Pos3idon]

if it is only 3/4 up, As soon as you shifted the cab control to lower the cab should have come down via gravity...

1. Where was it leaking?
2. Where did you add fluid?

I thought it would come down with gravity as well.

The leaking is from the hydraulic latch behind the cab (I've ordered new o-rings for this). I added the fluid to the reservoir behind the latch.

Your advice is truly appreciate!

 

[Third From Texas]

Sometimes it's helpful to add the fluid into the upper (shared) reservoir at the pump...then bleed air from the lower reservoir at the hand pump by working the pump. Repeat as needed.

 

[Ronmar]

If you are using the lower pump, you need to add the fluid to the lower pump. There is an allen head screw plug on the rear outboard corner. The control valve system is plumbed so that it sends returning fluid with a preference to the lower hand pump, so it always remains full/topped off. In theory the lower pump should be able to draw fluid from the upper reservoir but it has to do so thru a 1PSI check valve, so it must reliably pull a high vacuum to do so. I am not sure how well that works as hand pumps are notoriously poor at pulling a Vacume… so better to just add it to the hand pump.

The cab and tire cylinders have load check safeties to prevent the load from freefalling if they blow a line. There are restricted orifices in the control valve That keep the flow low enough to keep from tripping the safeties. if you pumped air into the line, this could have caused the return flow to briefly exceed that flow speed when the air pocket went thru the restricted orifice and cause the safety to lock. After you add some fluid to the hand pump, try and raise the cab again, this should reset the safety, then you can again try and lower the cab.

 

[Ronmar]

I guess if your latch leak is bad enough, that could also allow the outflow from the cylinder to be great enough to trip the safety in the cylinder base..

If it locks again when you try to lower it, you may need to disconnect and cap the line to the cab latch...

 

[Pos3idon]

Thanks for all the advice, I will try that this afternoon and let you know how it goes!

 

[Pos3idon]

Ok, so I traced the issue down to a blown gasket in the power unit blowing hydraulic fluid to leak out when the cab dropped down. I was getting ready to replace it and finally figured out the secret to getting the cab down was to move the switch from the lift to the lower setting EXTREMELY slowly to prevent the cab from dropping to fast and making the safety kick in.

After that I realized that the cab latch doesn't engage properly if you don't use a screw driver to push it the last tiny bit of the way.

On the bright side, I thing I'm starting to get a feel for this truck and what she needs.