square D wont cut out

I have a sqaure D 9013 fsg-2 installed on the main water feed to my tropical fish house (we are an Aquatic Wholesaler) and it refuses to turn off without the aid of tap with a brush or similar . I have had the cover off ,disconnected the power and squirted some wd40 onto the mechanism to no avail.Being a tropical house the air is very hot and humid and things tend to corrode quickly,I was wondering if the switch is knackered or mayb some muck blocking something to stop it turning off. The pressure builds up to the same mark as it did b4 but no action.
Any help wud b appreciated
Cheers

Try this; turn off power to switch, drain tank (relieve any water pressure), remove switch and conecting tube and clean any crud build-up. While your at it with the tank empty of water, check the psi in the tank to 2 psi below the pump's cut-in setting.
If after doing this, the pump still will not shut off, the points may be pitted (burnt). then its best to replace the switch.... $ 10.

"and it refuses to turn off without the aid of tap with a brush or similar "

Sounds like you've had to "tap" this switch a few times in the past. Could be the pump is just not quite making the pressure the switch is set for. Try turning it back a little, like maybe 1 - 2 turns and see if the pump turns off now.

When I tap one I use a pipewrench (just kidding)

You wouldn't be from Gibsonton, Fl. The tropical fish and carney capitol of the world would you?

bob...