Bobcat 435 mini excavator thumb wont hold

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

Good day
The circumstances you describe illustrate pressure loss in the neutral position could be the result of leaking shock or main valve.
If you dead end the thumb lines with a pressure gauge observing the pressure loss that may well pinpoint what is happening.
Switching the shock valve is a good first step to eliminate the components.
Kind regards
Uffex

Bad piston seal in the thumb cylinder on the push end of the piston? Try reversing the hoses if you can, see if it acts exactly the same except the switch will be reversed, or if the problems change directions.

I know nothing about the plumbing on your specific machine. I've had a similar problem.
It is common on thumbs to damage the thumb cylinder squeezing the bucket against it.
When you push the thumb against the bucket, system pressures might be limited to 3500 LBS.
When you push the bucket against the thumb, the thumb valve is closed, the relief valve won't limit thumb cylinder pressure. Manufacturers use "port relief valves" as a relief for this situation, perhaps 4500 PSIG.
Not a lot to go wrong with these valves, an o ring, or some other tiny component failure, or even a tiny piece of debris in it could cause leakage.

Good day Willie
The situation you have outlined is indeed common if the thumb & bucket cylinder is of a similar diameter the damage is less prone to occur. If the bucket cylinder can make the thumb cylinder reach a high pressure the effect is the same as a pressure intensifier.
One solution would be to install a larger relief to the thumb positive side, the standard port relief has only a very low capacity leaving the circuit prone to high pressure even with a low pressure rating.
Posted some information which may be of interest there is more on the web site should you like to take a look.
Kind regards
Uffex

Good day
Sorry overlooked this response doc.
Kind regards
Uffex

Just to follow up..It was the port relief valve. Was about 1000psi out of adjustment for some reason. I adjusted the rest back to factory spec while I was in there and it really woke the machine up. A couple were low by 500 psi or so.

That's a lot! Good job