L3750 priority valve repeated failures

[patchofpentaphyllum]

Late 80s Kubota L3750 4wd hydrostatic shuttle

Dec 17 pressure testing my mechanic found the source of my problems (low/slow/no loader power) to be my priority valve. we replaced it with an oem priority valve ($523). The old unit was non oem, this is at least the third priority valve. At that point I was concerned there was another reason for the failures

symptoms have returned less than 6 mos later, now worse, also with slipping hydrostatic shuttle (fwd/rev). total loss of power at loader and 3 point lift

I found where someone else had a problem with a similar priority valve and fixed it (http://www.orangetractortalks.com/forums/showthread.php?t=21591) by filing down the plunger within the valve

However in my priority valve, the plunger im pretty sure is non servicable. It is housed inside a plug. (photos attached of my priority valve diagram, actual non oem priority valve removed from my machine, plug containing plunger/moving parts of priority valve)

What are other causes for priority valve failures. I changed the hydraulic oil at the time of the last one. Could it be a pressure release valve issue? Looking for common causes of priority flow valve failures. If nothing else is there a method to bypass or install a priority valve from another model. Any suggestions appreciated.

 

[100 td]

First up, filter and screen have been cleaned/changed if it has one? Does it have wet brakes? What oil are you using, any contamination? I don't have access to parts diagrams, I doubt the orifice is available, worth a check?

 

[100 td]

Found a parts diagram, no serviceable parts listed, after doing filter and oil checks, also check the suction line rubber hose and clamps for tightness/damage/deterioration, then pull the priority valve and see if you can clean/disassemble check for debris, check for binding etc. From the picture you provided from your manual, it appears you will be able to fully access the valve. YMMV

 

[100 td]

If you have had any contamination in the oil at any time, particularly water/moisture, it could play havoc with this valve, as well as the shuttle valves I expect.

 

[100 td]

my problems (low/slow/no loader power) to be my priority valve.

less than 6 mos later, now worse, also with slipping hydrostatic shuttle (fwd/rev). total loss of power at loader and 3 point lift

After re-reading..........

So first time loader problem, now both loader and transmission, I reiterate my initial thoughts of oil flow problems, filter, oil level, suction line leaks, before pulling valves. May even be worth pulling the suction hose for a look, and I've just had a look at the parts diagram and of course you could have a failing pump or pump drive coupling which is a known point of failure on some models with this stepped drive arrangement.

 

[patchofpentaphyllum]

despite the similarity in symptoms and the repeated probs with priority valve this time it was a hose directly underneath the seat all the way down just above a skid plate, that had become old and kinked and was also full of debris, backing up the new hyd fluid and filter. replaced the hose and voila. prob still going to flush and put 2 new buckets of fluid in.

 

[patchofpentaphyllum]

update: the oem priority valve I replaced in the machine is still working and had never failed. The cause of slippage fwd/rev was because the f/r shifter was not being fully seated in either position. This was due to how I had it rigged, because the ball joint shift cable linkage has been disconnected for a while. When I put it back together it doesnt stay together so it was easy enogh just to run some cables to pull it into either fwd or rev.
The loss of hydraulic power was due to a kinked return hose underneath the passenger footwell. The machine is still operating great today.

 

[heppeone]

Have same model, L3750DT, similar problems. Will update once (if) I find source of problem.