I used to love the Sullivan Fuel Filler Valves because of their convenience and they never had leaks that I'd heard about with other valves. HOWEVER, when two of mine split in cold weather and popped apart causing ejection of the spring and ball, I removed them from my other planes.

The first photo shows an intact Sullivan Fuel Filler Valve and one that failed. You may be able to see that the housings are made of two different materials but both of them had small splits at the edge.
The second photo is from a Tower Hobbies ad and shows a third Sullivan valve material that appears to be black in color. If I asked in a very polite and gentlemanly manner, I wonder if someone who is good at making dead stick landings would try out that new Sullivan valve and report back with the results.

The third photo shows my well worn UCD 46 with a filler made from 2 bulkhead fittings and a piece of fuel tubing. This arrangement is probably infinitely more reliable than any valve but is not as pretty. It has similar advantages to the valve in that it prevents fuel from being pumped into the engine and it requires no third line in the tank.