Tricky business there.
The valve should be closed when the rez is low due to flooding your system, yet opens before the rez gets so low it can no longer complete the flood cycle.
Most float valves will operate correctly submerged. They still want to float and the float is usually closed.

I guess my concern becomes a backup...I need a fail safe. I found this system which could be adapted to my setup...interesting...

Here is a link that might be useful: Control kit

How about this? I use a float switch with an AC solenoid. This is hooked into the timer with the pump. When the pump is on, the solenoid is off (no added water). When the pump is off, the solenoid is on and would only operate when the switch told it to. I like it. The only concern would be the time after the pump turned off and the system was still draining. I think a time delay relay would solve that. Overkill?

I don't know how deep down inside the reservoir (below the full waterline) the float valve would need to be. But the only problem I can see is the deeper under water the more pressure pushing up on the float there will be. That pressure just shuts the valve. But under enough pressure it can bend the arm. In that case I see two easy fixes.

One initially bend the arm up enough so there will be minimal bending if at all later, and/or reinforce the arm. Second, drill a small hole in the float and fill it (say about half way up) with sand, then seal the hole back up. The less air inside the float, the less pressure pushing up when it's deep under water. Also the weight of the sand will counteract that pressure pushing up. You may need to play with it a little to get just the right amount of sand/weight inside the float though.

Float valves last a long time and it isn't like they are going to rust (being designed for use in water). But if you needed an absolute fail safe system. I would use two for redundancy. And give me a little time and I can come up with an automatic shutoff to protect from overflows, even though I would think it would be highly unnecessary if the float/s is weighted properly. Though just like any other part of a hydroponic system, the float valves will likely need cleaning from time to time.

If the metal arm of the valve is anything like you'd find in a toilet, 10 ft of water isn't going to bend it much (if at all). However, if you're worried about it, fashion the connection so the float pulls straight up on the valve, then no bending will occur.
bogaat,
yes adding electronics to it sounds like overkill. Though, it may prove the best way if you're good at such things. But then again, you have an issue with power failures. Of course, w/o power, your pumps won't work anyway.

I was thinking of an even smaller float valve and shorter arms, like the type found in portable swamp coolers. I have seen them at Wal-Mart for under $4 each. They are fed by a 1/4 inch tubing, so they don't have high volume. But even a 15 minute (pump) cycle, that could still add gallons of water with each cycle (for just one of these float valves).

So after designing a crazy bullet proof system with relays, I have fallen back to the simple. A float valve at the top of the tank with only a trickle of flow. When the level drops during watering, it will add very little...ideally, less than what the plants take. As my fail safe, I add an overflow to my tank going out of the greenhouse. Worst case, I see water flowing through the pipe and know something is off. I will have to play with the flow depending on my timing, but I think this will do. Thanks for all of the feedback!