Yep, you have a leak on the high pressure side. Probably in the
connection where the little hose is suppose to pick up and mix the
soap. I took that connection out of the line because I don't mix soap.
Just hook up the high pressure hose direct to the unit. That was my
problem anyway. Hope this helps.

Hank, It is no visible leak under the washer but through the high
pressure output. When the trigger is loosen in the hand gun, it is
still some liquid flowing out and as a consequence the pump restarts
to maintain the pressure. That is why I think the problem is inside
the pump...

Thnaks for yout hint
Sammy