Hallo from Bulgaria. I have the same water pressure pump. The question is old but naw I saw it. The problem comes from valve in this modul:
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situation: If you are change hose and gun, the nozzle in new gun is with small diametar. The water pressure pump have more debit from this who may come from nozzle. Then the pressure push the valve and valve close the holes for pressure of pump. So the pressure pump works pulses and the pump load. ?ou need to remove the moving part of the valve, but you must keep the bolt
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because it stopper the hole to low pressure or put other.
2 situation: Your original gun or hose (nozzle) there is a blockage. You have to clean nozzle and hose. Do not worry if you remove the valve. In this water pressure pump, valve is in stupid situation. It can not safe the pump or other components - on the contrary, it is placed to adjust to a small range by pressure knob at the front end of the machine. It work good but when all in the machine is original. When you ruduce the pressure the valve make pulses again but it is so small that you can not feel. But one thing is certain - when valve work, it difficult the pump... I am remove the valve many years ago and it has no problem. Sorry if my english is not vary well, but I am from Bulgaria. I hope I've helped, if there is still a problem!