Remove lid.along the top there's a round 'puck' with wires on one side and a thin hose on the bottom. This hose goes down to an air chamber moulded into the top of the filter housing. The air is forced by rising water up the hose to the pressure switch(the puck). The switch is usually fine, look for a blocked air chamber and/or hose(try blowing down the hose).May also be a hole in the hose letting the pressure escape.
The Phantom fixer., May 2006
