Komatsu PC290LC-11 no hydraulic or travel functions

I think you might be dealing with negative pressure as you suspected. Here's what I'd try to figure it out:

First, open one of the safety valves on top of the hydraulic tank to let outside air flow in. You could also try using an air compressor to push compressed air into the tank.

Then check the suction port of the main pump itself. I ran into the same problem once, and after troubleshooting for days, I found a foreign object was blocking the suction port on the main pump. That turned out to be the issue.

I was just asking about the suction strainer, not trying to criticize anything.

This is a pretty unusual one, but just to be thorough, you might want to try swapping out the main pump line on the machine. There could be an issue with the main pump #2, which is the front pump.

I wanted to give you an update on the PC-290 I've been working on. Thanks again to everyone who helped out. After I replaced the self pressure reducing valve and cleaned and drained the hydraulic tank a couple of times, plus changed the filters and strainers in the tank again, the machine started running fine overall. But now it's developed a new problem where the engine stalls when there's heavy hydraulic load. I traced it down to cylinder number 2 not firing, which came back as a faulty injector. That should sort it.

I've got another question for anyone familiar with the Komatsu PC290LC-11. On this same machine, when I'm operating the travel functions in opposite directions to turn, or running the arm and stick functions into relief, it randomly loses pilot pressure and sometimes stalls. When the pilot pressure drops, I can tap on the pressure reducing valve and get functions back again. The pressure reducing valve is new and I've removed, inspected and cleaned it without finding any issues. The fact that tapping on it restores function makes me think something's sticking, so I'm wondering if the merge divider valve that the pressure reducing valve bolts to could be malfunctioning. Has anyone run into something like this? Fluids and filters are good, and the other pressures look fine. Any ideas?

I've seen merge divider problems before, but only a couple of times and on older machines—kind of unusual. If it's stalling under heavy load and keeps doing it, I'd check the PC epc on the pump. That said, it does sound a bit like merge divider, though an arm in hydraulic stall can also stall the engine if the PC epc isn't working right. I haven't personally tried counter rotation when I've run into this fault, so I'm not sure if that could cause it.

Machine's available again today, so I'm going to pull out and look at the merge divider. I'll also test the PC-EPC. Thanks for the help.

Have you figured out what's going on with the pilot pressure disappearing? I'm dealing with the same thing on a PC220LC-8.