If you are going to use a common feed system, the best way to do it is via a Left/Right/Both/Off type selector valve.

With that system if you are running on “Both” and you have an uneven feed, it’s easy to resolve, just switch to the fullest tank.

More important is the missing fuel cap scenario. Most aircraft have a vent tube that is situated is such a manner that it works like a little ram air scoop to force air into the vent system.

Most pilots think that when you loose a fuel tank cap, the fuel is sucked out ….. not so. What happens is the fuel tank without the cap fails to pressurize the way the fuel tank with the cap does. As such (in a “T” or common feed system) the higher pressure tank begins to push the fuel into the lower pressure tank. Of course with no fuel cap, the low pressure tank overfills an the excess fuel goes overboard.

Once the last of the fuel passes the common junction, you have fuel starvation and one full tank of fuel that you cannot access. In a system with the described selector valve, selecting between individual tanks would make this a non-event. If you were running on “Both” you could empty the high pressure side but once it started sputtering you could be back in business by switching to the full tank (with no cap) and would still be able to access that fuel.

Just something to consider. I just picked up n airplane this past spring that had a common sump system. The first thing I did was convert it to the system I described above. The owner said it didn’t feed from the wing tanks evenly and he would have to land sometimes to let the fuel settle out to an even level then press on. This selector valve setup allows you to correct the issue in the air.

T Mann