I recently had the same problem with my old Ryko Power Wash IV equipment.
I diagnosed using the following approach:
Feel the temperature of water weeping out of the gun to see if it is hot or cold. In my case the water was cold.
I then closed the cold water feed to the bench and see if the water stopped. In my case it did.
To make 100% sure this was the issue I also disconnected the supply hose and turned the cold water feed on. In my case it started dripping out of the hose, so I was pretty sure that was the issue.
I replaced the 1" solenoid feeding the pumps. I can get you a part number for the Parker solenoid tomorrow if you like. My equipment uses 24VAC solenoids.
Attached picture of my wash equipment. Don't judge too harshly, I have since done a lot of work cleaning this bench up.