Probably because your pressure valve needs adjustment.
It's white it's under the seat most c/vans nr the cold water inlet feed pipe. Turn the knurled knob on top clockwise to close down. Turn on the cold tap in the kitchen sink. slowly open the valve again untill you hear it clicking STOP turning the knob. Turn your tap of the pump should stop.
I have to do the same occasionally in my Lunar.
Bob
October 2009