There are some good ideas in the above posts, but I'm going to try to give you insight into your original question.
1. What you are worried about is the DIFFERENCE in the pressure on either side of the valve, not the total pressure. If you use a design that is set up for a 3-5 psi pressure differential, make sure that you have a recieving vessel for the water that is pressurized to the same pressure (or a little less) as the main tank. In this way you can make a cheap valve do the work of an expensive one (old engineering trick). This is called pressure referencing. You could drain the main tank into a small air reserve tank, for example, then at regular intervals shut off the smaller tank from the larger and drain.
2. Typically, any floatation valve that uses a ball inside of it uses a ball that is hollow - ie very sensitive to differences in pressure. If you replace it with a SOLID ball of plastic, the ball will still float - although it may sit deeper in the water (less bouyant) so it will tend to open later than when the hollow ball will. Solid balls won't collapse under the pressure difference, hollow balls will.
