Hi, I need some advice from someone who knows pumps......

I have a koshin 1" water pump which supplies water from a large mobile trailer mounted tank to an attached pressure washer. The whole set-up is trailer-mounted. The pressure washer has a small capacity internal tank with a valve which cuts off supply when the water in the internal tank reaches a certain level. So, my question is this - Do I need to fit a pressure relief valve to redirect the pumped water back into the large tank when the small internal tank fills up and the valve closes? Or can the Koshin water pump cope with the water-out being stopped by the valve in the pressure washer tank?

Any help gratefully received.