RO/DI Unit Problem

[Demolite]

I have owned the Maxxima HI-S 35 Gallon Ro/Di Unit for about 7-8 years. I have changed the filters and the membrane at there correct intervals though the years.

The filter is producing water O.K. but recently when i turn off the in line tap to the supply the pure water stops flowing but the waste pipe continues to drip.

Any idea how i can stop the dripping ?

Thanks