Under the cab, there is a pump with a pressure sender hanging out of a connector. These go bad and can keep the light on.
If it's the style with a big fat hose that goes to a block on the left side, the sender will be in the elbow where the pump starts to be covered by the chassis where the fuel/oil tanks sit. If it's the older style with a square block on the pump and 3 hoses on the left. The sender will be mounted into this block.
Another test is to remove the sender and install a pressure gauge. The pressure should be 90-120PSI from memory. If it's a lot lower, don't run the machine long, something is wrong and may cause more damage.
