JD 440B hydraulic issues

[pmcferron]

Long time member, first time poster.... I have found myself in a bit of a pickle and Ive thrown a lot of money at a problem and still have yet to fix it. I have a 440b with the synchro shift transmission. When operating the skidder it does fine until you try to use 2 hydraulic functions at once, doesn't matter if it's steering and blade, brakes and blade, steering and brakes no matter which 2 it will completely lose function. It you use any of them individually it works completely fine. Here is a list of things I have done.

Fixed leaky lines and cylinders
Changed trans fluid and filter
Cleaned the trans pickup screen
Replaced transmission pump
Disassemble and clean priority valve
Disassemble and Clean relief valve just beyond the priority valve.
Main front pump replaced
New brake pads

All of this to no avail, any suggestions aside from what I have tried? I'm desperate for help at this point, I can't figure it out.

[Southside]

I had similar issues with my 440A, after testing the spring strength on the priority valve and setting everything into spec it still was a problem. Turns out the shuttle assembly wears the valve body enough they all fail so I gutted mine and the problem went away. 

[pmcferron]

I had similar issues with my 440A, after testing the spring strength on the priority valve and setting everything into spec it still was a problem. Turns out the shuttle assembly wears the valve body enough they all fail so I gutted mine and the problem went away.

So gutting it will not cause it to do the exact same thing it's doing? I was afraid if I gutted it then it would act just as it is acting. When you gutted it did you have to plug the small drain back line that goes to the trans? 

[Southside]

Mine ran fine after gutting it and yes I blocked off the case drain. My steering wheel would shake back and forth violently when it got really bad, that you stopped.

[mudfarmer]

When I gutted mine it was no bueno and had to put it back together, the thing is unsafe even when working properly 

Curious what you find if you pull the trans filter and screen again, and what you found last time. I still don't know what my original problem was but it was fixed after another filter change and -putting the filter back together right -

One thing is you can gut the priority and if it doesn't fix it you know it isn't that, it's quick and easy.

[East ky logging]

I gutted my valve in the same skidder that I used to have after reading on here what southside did to his. It helped some it was where the blade wouldn't raise if you was doing anything else. I also capped the drain off. Good luck working on the worst hydraulic system that was ever put in a piece of equipment. 

[pmcferron]

So I gutted my priority valve and the thing is actually worse now then it was before. I've studied and thought and pondered and wonder is it possible the hydraulic filter relief valve would be stuck open and be letting fluid bypass and not come to the front main pump? Or is that even possible? I put a gauge on the main pump and it reads roughly 2400psi but if I move the blade it drops to like 500psi???  Would it be starving for fluid causing this? 

[mudfarmer]

I cannot remember (much of anything). In the manual there is a good section on systematically testing hydraulic flow and pressure. You can buy PDF copies of the manual online, and it is a very good manual 👍