1: The air filled can is full of electronics, filling it with water each time i place the cap kind of defeats the purpose of needing to keep the water out.
2: We do a vacuum test to check the rest of the seals, but having a port that i can use to induce air means i have another hole to check and effectively doubles my problem. Then i would have to make sure two caps are in place.
@1: this first reversal step was just to generate ideas 
It brought up the idea of instead letting something flow in (which was the problem) to detect a leak to let something flow out (solution) ==> pattern solution = reverse(problem) - ok not allways but it helps to generate ideas.
@2: you don't need an extra hole, on contrary I would propose a small compressed C02/air cylinder, a valve and a pressure sensor and some bits to control it. Yes it means some additional electronics. After the cap is closed the valve opens to raise the inner pressure by X%. If there is any leakage wherever it will be detected in contrast of just detecting a switch has been pressed the right way.
Additional, if there is a very small leakage some overpressure might prevent water leaking in (keep it local)