Power shower pump will not switch off!?

I have an upstairs power shower. When switched on a pump downstairs automatically switches on and increases the preasure to the shower. This pump is suppossed to switch off automatically when the shower is switched off but it dosnt.

As a result the presure builds up and the pump starts leaking. Does anyone know why the pump isnt switching off and how to go about fixing it?

Jonathan
June 2006