Please send me a picture of your box. There were a few in the past that have done this due to a casting flaw in the control head. The newest boxes have a different casting and I have not had this happen in some time now. Since light effort is not normal with these boxes,, I would like to know what it is to start with. However, it is possible too much relief pressure or too much flow is not helping the issue. These boxes like approx 1400 psi as opposed to the old boxes liking 700-1000 psi.
