Hi , the most common cause's of this fault is a deffective unloader valve or a small peice of dirt or grit caught in the pump valves , unless your quite mechanically competant i would suggest you take it back to your local Karcher service agent as to repair them your self is quite a involved process , if you want to try a repair yourself this link is quite good
http://www.paddockdrayage.com/Karcher/index.htm