Hey Dave is this what you need?
http://www.jlaquatics.com/phpstore/s..._ID=ro-kpfshut
My RO has a float valve in the reservoir (50g barrel) that stops the flow of RO when full and the valve turns off the water flow through the RO unit.
To get fancy you can put a second small tank on top of the barrel (in my case) such as a red sea 5gal jug (came free
) that goes to the sump float valve. Put a PH (MJ on a apx 20min/24hr timer) in the barrel that pumps, for a pulse each day, RO into the top tank.( An overflow line back to the barrel allows this to be a crude time setting)
...the idea is to fill a days worth of RO at once so the RO isn't just slow leak all day but runs open for ,a time, once per day, after the PH fills the small jug then stops.
Hope this is clear and helps