B1700 HST hitch leak-down when stopped

[DaveD]

The 3-point hitch leaks down at a very visible speed to the bottom when the tractor is off and the height lever is set at 5 or below. It will, as normal, quickly go down to the setting height (depending upon the position of the lowering speed adjusting knob), then slowly leak down all the way, perhaps in a minute or so from the 5 position. There is no leak-down, even overnight, when the hitch is all the way up or set down through about the 6 position. It does it hot or cold, and when the mmm is either engaged or pinned all the way up, in which case the only weight on the hitch is the quick hitch, about 70 pounds. It's not the piston, as I closed off the lowering speed adjusting knob and it stays put in whatever position it is in when this is shut off.

I have the service manual and access to Messick's parts diagrams. From my study of the problem it appears that it is probably inside the hydraulic cylinder case, perhaps in the spool/poppet balance link external to the control valve, or in the control valve itself. What baffles me is that it leaks down only from the lower positions--if the balance mechanism isn't working properly it seems that it would leak down from any position. The external feedback rod is not worn and is lubricated and properly adjusted. When the tractor started with the height level undisturbed it will quickly return to where it should be.

The 1996 B1700 tractor has 1260 hours and has never slept outside, but always in a tight shed. I keep it clean, well lubricated and the hydraulic oil is clean and full.
I can service the tractor mechanically, as I have done all the work on it for the last 25 years including many years ago separating the tractor in two at the HST/transmission case to replace a blown gasket. But I would rather not do something I maybe can fix externally.

Thanks in advance for any thoughts and ideas about what may be causing this.

 

[rbargeron]

The likely cause is a failing (or failed) o-ring on the piston in the lift cylinder.

There's a very good discussion available here with several ideas on getting the piston out without taking the top hydraulic assembly off the tractor.

Good luck - Dick B

 

[DaveD]

Dick B, thanks for the suggestion. However...

This is a B1700 and the hydraulic unit does not have a separate head that can be removed for access to the piston. The whole unit has to be removed--not that this makes much difference, as I don't believe removing the hydraulic unit would be such a big deal. It's just that I would have to remove a lot of stuff off the back end of the tractor and move a lot of machinery in my shop to get the tractor below the chain hoist, since I'm getting old and can no longer wrest something as heavy as a hydraulic unit off, even the B1700 version.

But it doesn't appear to be a failing or failed O-ring in any case. As I stated above, if I close the lowering speed adjusting knob at any position, including a position below 5, no leak-down occurs, and since the adjusting poppet valve, actuated by the adjusting shaft, is coupled directly to the cylinder exhaust port at the top of the cylinder, oil cannot be escaping past the O-ring when it leaks down, and that pretty much eliminates that part of the unit.

So again, before I tackle this job I would like to discover that it might be fixed without going inside the unit.

Dave D