Hoping someone may have some insight. I'm having issues with the boom dropping with even the slightest movement of the joystick which makes fine grading an absolute nightmare. Boom up doesn't seem to be too much of an issue though. When working in combination with another function it drops slightly less. The unit has seen some stuff it was a dealer rental that was stolen then recovered years later and sold at auction, which was bought by the person who I bought it from. The valve bank had been taken apart and then put back together by the previous owner and then I took it out and had it completely rebuilt by a reputable hydraulic service company. All other valves work as they should. Any suggestions would be greatly appreciated as I'm at a loss.
Has the cylinder been checked? Will it lift and hold the machine ?
I had this issue on a Kobelco excavator years ago after a pilot line to the joystick let go. There were screens on the base of the joystick where the line attached and it had debris stuck to it. Took the new line back off and cleaned the screen and all was good. Not sure that’s the case with yours, but it may be worth a look since you don’t really know what’s been done to it in the past.
I was thinking along the lines of joystick too. Does it have a diverter valve to change the control pattern? If so switch it over. That'll swap the joystick that controls the boom to the other side.
Boom cylinder will lift and hold.
It does but same issues.
The above suggests the pilot system/joysticks and boom cylinder are operating correctly. So with the boom raised off the ground and no joystick commands, the boom holds ok? That would rule out a line relief and boom drift reduction valve. Edit: the way they usually fail anyway. I'd be looking into the main control valve unless someone has a better idea? Sticky spool or broken spring maybe?
The boom does drift down. Its slow but it does drift.
I also had the valve bank rebuilt
Does it have an anti-drop valve fitted to the boom cylinder (usually in the shape of a block mounted on the cylinder itself), aka lowering control valve? I think you're going to have to check pressures. Would like to know if the pilot pressure is within spec. No specs here unfortunately.
If the issue can’t be found, it may be reasonable to plumb an anti-drop valve or holding valve into the boom down circuit. It may have been there in the past but taken out in its travels. Need a hydraulic circuit spec.
Here's a few pics of the boom valve. No signs that it had a anti drop valve installed at all.
Update. Pilot pressure and circuit pressures are right on spec. I also swapped pilot hoses from bucket to boom and the boom still has the same issue so the rules out the control/joystick.
Could you possibly swap hoses to the cylinders? Almost like a pattern change but physical swapping hoses.
Update! Fixed the problem. I discovered the spool on the boom valve was installed upside down. This even with having the valve bank completely "rebuilt" by a "reputable" hydraulic repair company. $8500 and they couldn't figure that the spool was upside down... Along with pump 3's relief valve being on the stick valve needless to say the machine runs like new now And the old saying "if you want it done right, do it yourself" continues to be true.
Update! Fixed the problem. I discovered the spool on the boom valve was installed upside down. This even with having the valve bank completely "rebuilt" by a "reputable" hydraulic repair company. $8500 and they couldn't figure that the spool was upside down... Along with pump 3's relief valve being on the stick valve needless to say the machine runs like new now And the old saying "if you want it done right, do it yourself" continues to be true.
Don't have much to contribute to this thread other than bummer about solving your hydraulic problem yourself but I do like your matching paint scheme!
Thanks bill! I color match both to a M1079 i owned for several years. Between the color and the cab over truck it turns a few heads.