How can I fix the pneumatic switch on my Jacuzzi J-Twin 5-Jet bath?

I have a 1999 Jacuzzi J-Twin 5-Jet bath and the pneumatic on/off switch has been getting harder to use and is now ineffective. The resting point of the user on/off switch has been sinking lower and lower, and now pressing its remaining travel does not activate the motor (pump) switch. Can anyone please suggest the likely cause and remedy?