Kubota M8540 auxillary control valve leaking

tuolumne · Jan 14, 2021

For a while I have had oil oozing out around the plate #150 in attached diagram. There is a pinhole drilled in the plate and now oil always comes out of it while that circuit is in use, and sprays out when the relief valve trips. What is going on? There is no visible O-ring when the plate is removed. What is the purpose of the pinhole? Where is leakage further back in the valve that is allowing high pressure oil at that location? I would appreciate some input before I tear things apart further.

The full valve is visible at this link http://kpadweb.kubota.co.jp/kpad2/PartsInfo.do?bookCd=100K1525&funcCd=&figNo=&himban=&dispType=0

oldnslo · Jan 15, 2021

The Kubota link did not work for me but is item R a spring and item F a piston or spool. My suspicion is the pin hole is an atmospheric vent to allow that piston F to move. If my suspicion is correct the seals on item F are leaking allowing oil into the spring chamber

tuolumne · Jan 21, 2021

oldnslo said:
The Kubota link did not work for me but is item R a spring and item F a piston or spool. My suspicion is the pin hole is an atmospheric vent to allow that piston F to move. If my suspicion is correct the seals on item F are leaking allowing oil into the spring chamber

Piston F is hollow inside, so no vent would be necessary for its movement. I have the whole assembly in pieces and cannot find any visible damage to o-rings or moving parts. However, I did notice that the 4 ball bearings (W) are missing. Previous owner had it apart?? I am not sure what their function is other than possible centering. This could allow the shaft to drift a bit and leak?? However, they have obviously been missing since before we owned the machine and the leaking has been getting worse. Also, they do not appear to be available as a replacement part. Here is a whole picture. I will try to replace the oil seal "040" to see if that solves the problem. As far as I can see that seal is the only thing preventing high pressure oil from moving back into the block "C" Any comments? I am way over my head!

oldnslo · Jan 21, 2021

What does Kubota call items I, S, and H? It looks like there is an adjustment screw that holds them in place.
Does the hole in the cover look like it was drilled intentionally or wore through?
I would look at any passage that could potentially allow oil to enter the chamber under the cover. I believe should just be atmosphere since any pressure on that end of the spool would tend to push it out the other end like a single acting cylinder.

Leejohn · Jan 21, 2021

Could that valve be a hyd control detent? I would also say #40 is where the leak is coming from. What do they say #60 is?

tuolumne · Jan 23, 2021

Having this apart, I agree that it is likely 040 that is leaking. It is an oil seal. Inspecting the unit when I took it apart it seems that those 4 ball bearings, W, are critical for proper centering of the shaft as it moves. 040 probably wore out for this cause. If I replace the seals I'll be in the same boat again after a while. It actually got worse quite quickly since our application was moving this valve regularly. The previous owner may not have moved it as much. The bearings are not available and Kubota can't track down any specs on them so I just ordered a used valve to replace the one that's leaking. $650 is better than $1400 for new. Hopefully. Thanks for your input everyone.