The hydraulic system is load sensing for the bucket, stick, boom and travel circuits. Swing and blade are on a separate pump. There is one main relief valve for the circuits on the main pump. Load sense pressure shows up at the pump when a function is operated and the pump output rises until pump pressure is about 300 PSI more than what is being felt at the implement function. Basically if you put a gauge in the bucket cylinder and one at the outlet of the pump, you would see 300 PSI higher at the pump than at the cylinder. That would be pretty straight forward if it wasn't for some other functions in that system. The boom, stick and bucket also have pressure compensated lift checks. What the lift check does is allow for a difference between the pressure in the circuit to equalize with the pressure at the control valve spool before it opens up. If the lift check wasn't there the implement, say the boom, would drop slightly until the pump pressure rose and increased to lift the boom. A further complication to all this happens when you operate more than one function at a time. Lets say you lift the boom and open the bucket. The boom takes a bit of pressure to lift but the bucket only really needs gravity to open. Without the lift check the boom would actually drop because the pump flow would go to the least resistance. The lift check prevents that from happening. Going one step further now we can tie the outlet sides of all the lift checks together with a small pressure cavity. Now the lowest pressure function will have to come up to the highest pressure on the pump side before oil can supply that low pressure item. Now all the functions can work at the same time even though the bucket is falling open on gravity, the stick can fall on gravity and the boom needs 2,000 PSI to pick itself up.
Now comes the complicated part. You take that oil that works the pressure compensated lift checks and you also tie it back to the pump and now you call it load sensing oil. The pump is also using it as a reference point to stroke and destroke the itself. If we go back to those valves we can find there are seals installed to hold the working oil pressure against the back of check valves. If one of those seals fails that load sensing oil pressure will fall to amount of pressure that can be held at the leaking check valve. The pump will only put out enough oil to maintain the differential pressure. Lets say you want the boom to raise and it takes 2,000 PSI. You pull the handle and the spool moves and the pump puts out enough oil to lift the check valve but the work pressure side of the check valve is leaking off enough to only show say 1,800 PSI so the pump logic says it doesn't need a lot of flow and never puts out any more oil even though you open the spool all the way. Sound familiar?
So to answer your question, since the functions affected only involve the boom, stick and bucket you need to find out where the compensators are and see if there are seals installed on the components or if all the valves need to be replaced completely. I've looked at one of those machines and goofy tipping cab for access to that main control valve. Hopefully the compensators are just some kind of cartridge but I'll bet that access to them is going to be a pain.
Good luck!
