Kent R.o. System — water leaking

Ibought Kent grand mineral r.o. water purifier on 13/11/2007 It was working fine for the last 8 months and now it is not automatically shutting off when the water tank gets full I have been compling and still my problem is not solved the person who serviced last informed me when system is not shoutig off aske me to give a bang near the shout off switch and it will stop and this is only working.This is the state of the product.Now at night I have to shout off the system other wise the in the morning the kitchen will be flooded