I would suspect that a lift cylinder is leaking internally, on the piston. meaning the hyd oil used to lift (extend the cylinder) is going past the seals on the piston into the other side of the cylinder and returning into the tank, never letting the pressure build enough to lift the boom when loaded, but not leaking bad enough that its still letting it raise when empty. It happened to our 2600 once, you can't tell it from the outside it there is no external leak. You should be able to check for this by loading the bucket with something heavy, like something you can't even lift, then take the hose off on the rod end of the cylinder, then see if oil sprays out of the fitting on the cylinder you just took off. If the load cant be lifted (the cylinders are overloaded and cant move) no oil should come out. If oil does come out, that should be the bad cylinder needing seals. The oil coming out is the oil going past the seals that are supposed contain the oil for extending the cylinder. The seal kits are cheap, we did this a couple years ago and got new seals from Baum Hydraulics where about $15 or $20 bucks with shipping. I hope I explained this well enough to understand, good luck. I think this will be a cheap fix for you.