hyundai 290lc-3 hydraulic problem

[kenquip]

working on a hyundai 290lc-3 long reach.
the cylinders won't hold a load, they drift down. originaly thought it was the cylinders, rebuil all four of them. that wasn't the problem. the main boom cylinders act like they have air in them (bounce). with the arm fully extended when you crowd in it seems to cavatate.

any suggestion on where to start looking?

 

[uffex]

Good day ken
I would suggest that the bounce is not so unusual with a long reach rig, these machines have a additional load holding valve on the boom circuit this will lock the fluid in the positive cylinder side however there is always the shock valve can leak and cause the boom to creep. The additional load imparted to the shock valve from long reach unfortunately can result in this experience. It is possible to increase the shock valve pressure but maybe this will not be the end of the problem. Hope it helps.
Kind regards
Uffex