Well I have decided to stop living on the edge and add some redundancy to my auto top off system.
Right now it consists of a float valve in my sump being fed from a 1/4" bulkhead around 5" below the water level in a 30 gallon rubbermaid trash can I use as a fresh water reservoir. The reservoir has a float valve that is fed directly from my RO system. With this set up if the if the valve in the sump failed I would potentially end up with my RO system pumping water into the sump.
There is a bit of safety built into this set up. It was actually inadvertent that it worked out this way. I emptied out the reservoir one day then the next day I noticed that I did not have any top off water making it to my sump. The inside of the bulkhead in the 30g has a 10" long pieces of hose that aims down and if the water drops to far the hose will lose syphon. If it loses syphon it generally does not start back up on its own, I have to suck water back down the line. I would not trust this to work 100% since the bulk head is below the water line the syphon could start back up on it's own.
I guess to make it 100% safe from to much water making it into the sump I could put the bulk head above the water line in the reservoir and run a hose 10 inches of so under the water level then start the syphon, if the water in the reservoir ever fell below the hose it would lose syphon and could not start back up on it's own since the hose would come into the reservoir above the water line. Set up this way I would never get more then a couple gallons of water into the sump in the event of a float valve malfunction. The down side would be if I was out of town and the syphon failed the tank could go days without being topped off.
To remedy these issues my plan is to simply add a float switch slightly above the level of the float valve that is connected to a solenoid. The solenoid will shut off the water going to the sump in the event the the float switch is triggered. I will probably add a timer to the solenoid that will only be on for maybe 15-20 minutes every couple hours to allow my RO system to run for longer periods of time to avoid TDS creep. I guess potentially both the float valve and switch could fail but I think that is fairly unlikely
I will probably leave the bulk head where it is in the reservoir but remove the hose to avoid a syphon and the chance of it not starting back up.
I was looking at McMaster-Karr at solenoids and was wondering what my best option was.
http://www.mcmaster.com/#7877k5/=hq790
or
http://www.mcmaster.com/#5489t41/=hqndg
The first one is 1/8 but I guess I could get an adaptor. The second one has 1/4" quick connects built in but is 2 times as expensive.
I also need a float switch and was wondering if anyone had a suggestion for a good one. I think I will just go with a 120vAC solenoid so I need a switch capable of switching AC power.
Any critiques on my idea or suggestions are welcome.