Seems to me you have what's known as a pressure switch in the cold inlet pipe and these need adjusting time to time.
Check where the inlet is [under seat] if it has a pressure switch then it's white with knurled knob on top. Turn on the system and turn this knob clockwise to close the switch. Turn on a cold tap then turn the knob anti-clockwise until it starts 'clicking' STOP turning the knob. Then turn water off the pump should then stop working. Mine does this about every so often and this is the procedure I have to go through. Post an answer to your question if it works.
Bob, May 2009
