My pressure washer is electric (Stihl), so I have an additional element for running it: a
pto alternator. I put a 110gal horse tank in the front end loader along with the washer, then run the 115v cord back to the genny. I can add weed killer, tree/moth spray and soap, etc to the tank and wash anywhere the tractor will go. My washer has a water volume control via the pressure setting. And, there's no problem when and if it goes dry. Why don't you just throttle down the washer to use less water flow rate? Also, don't use it with a continuous spray. Just use it intermittently. Yes, I wash the tractor with the pressure washer, but not with full concentrated spray, I set the wand for a wide pattern and get the mud off. The major concern I would have is blowing water into the engine oil and hydraulic outlets, thus contaminating the oil supply.