Never worked on a Komatsu before any help regarding why the wheel loader steers heavy would be great. Customer already replaced orbital valve.
Pump losing pressure? Weak relief valve? Cylinder leakage? Check the easy stuff first, Does it have supplemental steering from a transmission mounted pump. Some times those setups have check valves that can leak.
I checked pressure from steering pump to priority valve. Only getting about 600 psi at high rpm's. Not familiar with this pump set up.
And it doesn't have an emergency steer pump.
The common setup for the larger Komatsu loaders used the steering pump to supply steering first and any flow not needed for steering was dumped into the implement circuits. When I worked on them the steering valve was mounted in the front frame on the left side. All the orbital valve did was move the spool in the steering valve. In order to check the pressures you had to pin the center link to the frames and then steer the machine one way or the other to check pump pressure. It wasn't uncommon for the pumps on the older machines to get weak or spin the bushings and the gears would start eating the sides of the pump cases. The pump cases were made of aluminum and those steel gears would take out big bites. How many hours on this monster and have there been any other pumps replaced or were showing problems.
Loader will lift itself off the ground so that's a good sign. Machine has just over 4000 hours.
Lifting itself off the ground is the implement circuits which do not interact with the steering circuit. Only the flow not needed in the steering circuit is available for the implements. It doesn't go the other way. If it has the aluminum bodied gear pumps I am familiar with, the service life I had observed was between 4,000 to 6,000 hours.
is steering left and right heavy? does engine rpm change the feel? theres a demand valve mounted on the rear frame under the orbital valve about 6" back. i would suggest removing it and cleaning it and the relief valve very well.