HISSDC
Registered
Some of the other types may not be but the classic cone shaped ones with the oring are water tight. Properly installed and if the dust cap has an oring it's installed properly and in good shape, they are water tight....actually they use the same type seal as a tank. That said, it's not uncommon for the orings to be missing and unnoticed.
Now I see. I have some dustcaps that are plastic and bad at waterproof.. so the o ring was missing. Thanks for telling.