Bobcat 435 Mini excavator thumb wont hold

[WyoJake]

Hello all. I have a new to me bobcat 435 mini excavator. All hydraulics work well except an issue with the hydraulic thumb, and I would love to hear your thoughts before digging into it. The thumb functions fine via the toggle on the joystick. Hit open and it opens all the way (and stays there, no drift...this makes me think the cylinder seals are okay) Hit close and it closes. But close it and release the switch and it has hardly holding pressure at all. The bucket will easily push it back in like the only resistance is the weight of the thumb itself. In order to pick anything up with it, you have to keep the switch held on close. Also, if you let the bucket close the thumb, when you open the bucket back up, the thumb drifts back down to where it was with no resistance.
Im probably not doing an great job of explaining the symptoms, and will gladly keep an eye on this thread and answer any questions you helpful folks may have. Were still using the machine, but it would sure be nice to not have to hold the switch to use the thumb.

Any help or ideas would be much appreciated. Thanks all, Jake

 

[SSdoxie]

There is a circuit relief in the auxiliary valve spool for the thumb cylinder base end to relieve pressure to protect cylinder rod bending when bucket curl force pushes against it.
Sound like that relief is weak, leaking.

 

[laurencen]

I would Tee a pressure gauge in the line and see what the clamp pressure is and what it hold at when the switch is released, also curl bucket and see if it makes pressure