Rockshaft slowly sinking (70D)

[DrCharles]

My 70D has always had a (very) slow sink on the 3-pt hitch with 6' bush hog attached. But in the last few years it has begun sinking faster, although it has no problem picking the entire hog up in the air.

But now I have to "bump" the lever to lift it, every couple of minutes. Time to find and fix the internal leak. It's not losing hydraulic fluid.

I'm assuming that I have to pull the entire rockshaft housing first. Does anyone ever get lucky and it's the control valve leaking pressure, rather than having to rebuild the piston/cylinder?

Any other tips/tricks welcome. thanks.

 

[r.w.b.]

Sorry but id say piston is culprit.

 

[DrCharles]

OK, I figured as much... so once I get in there, how do I repair the cylinder?

I doubt that there are any new parts available, so will I have to have the bore sleeved?

 

[Mike M]

I'm thinking the rate of drop adjustment can also be used to shut it off from dropping. So you can raise it and then close this valve and see if it keeps dropping. If so it is likely the piston o-ring or worse yet the cylinder gasket ? IIRC you can access the o-ring without removing the rockshaft housing. I was able to do it on my 50 and I think the 70 is set up the same. o-ring should be available from JD or other places that sell O-rings.

 

[DrCharles]

I'm looking at the parts catalog page right now (PC0445, Group 75-6) and don't see how it would be possible to remove and reinsert the piston in the cylinder without taking the rockshaft housing off the tractor, then removing the cylinder from the housing.

Could it be reached through the hole left by removing the control valve? That's the only way I see to pull the piston, if any.
I suspect the o-ring, a rubber part that's subjected to continuous movement, high pressure, and is over 60 years old. Rather than the gasket which seals two closely machined surfaces...

 

[Mike M]

You can unbolt the power-troll hyd. control box and then pull the piston out the rear. The rockshafts will have to be slide out a bit to allow the piston to come out.
A fellow named FredMo I think had a lot of pictures on his 60 showing this.
But if it is the cylinder the complete rockshaft will have to come off. Good chance it is the o-ring since it is slow leak.

 

[PJH]

About two winters ago I repaired the PowerTrol on my JD 50. I found the cylinder in excellent shape. That cylinder does not wear like an engine cylinder. I replaced the piston o-ring and the paper backer. Before the repair I was losing hydraulic oil into the final drive. Some of the piston bolts were finger tight, and none of them were safety wired. That's where it was losing oil. You say you are not losing oil, so I would think the cylinder gasket would have to be in decent shape.

Like Mike said, after removing the valve body, you'll have to pull the rockshafts partially out to access the piston. Before you pull the rockshafts, take good pictures and maybe even scribe some lines so you can get the rockshafts clocked in proper position when it goes back together. I failed to do that and I put the rockshafts back together thinking it "looked right", but nooo. I had to take it back apart and move everything one notch to make my three-point usable. Hopefully you can benefit from my mistake and have better results with just one try.