We had portable mounted pressure washers on a trailer with a large poly water tank which we used to drag around the flight line to wash down aircraft with, and it had a separate feed pump to provide water from the tank to the pressure washer pump. We did not use all that high of water pressure either at the PW, and it still required a separete feed pump to supply sufficient water. My pressure washers specs require a minimum of 12 gal per minute at the water inlet. Thats equivelant to about a 1/2" water line running at 60 psi to deliver that amount of water in 1 minute.
I am still curious as to why it requires that much water in a minute though as its rated at something like 4.5 gpm max, so the water sosurce is a lot more than what is supposed to be delivered from the PW itself.