How can I fix my tap?

I can only offer the following having never changed a tap microsiwitch but had 'Pump run on' problem.
I take it yours is the external barrel and pump with pump connected to crystal filter unit on side of caravan. If so check under seat where the crystal unit enters the c/van and sse if there is a white coloured unit [ it's a pressure switch ] in the water line it has a knurled knob on top.

With 12volt system on turn this knob clockwise to close it fully. Turn on the kitchen cold tap. Turn this knob anit-clockwise until it starts to 'click' and stop turning knob. Turn off tap this should shut off the pump. if not turn on cold tap and turn knurled knob on switch a bit further then turn cold tap off again, the pump should stop.
TO SEE A PRESSURE SWITCH.
www.caravanaccessoryshop.co.uk next to logo on top line is a search box type in it whale pressure switch.
when picture shows click on it for a larger picture.
It might be helpful if you post what the this and that is in instructions.

bob
September 2009