With all this discussion of valves and venting, I'd like to throw this idea out here.
In the interest of developing a more efficient 29mm hybrid motor, I wanted to find some way to automatically close the vent when the flight tank is full. The design I've come up with is a compact float valve that screws into the forward tank bulkhead, see attached image.
When the tank is nearly full, an extremely lightweight aluminum float slides up a slotted push pin until the flow path through the slot is blocked. At this point, the float continues sliding upward and compresses the spring until stopped by the boss just below the threads. When the tank is depressurized, the spring will push the float back down and re-open the vent.
To use this vent valve without compromising safety, the GSE has to incorporate a fail-open dump valve of some kind. My current design has a pneumatic dump valve that was inspired by the Half Cat valve:
https://www.halfcatrocketry.com/half-cat-valves
The idea is to have the dump valve connected to a long air hose that runs all the way back to the control station, where it can be safely depressurized even in the event of a total electrical failure.