Drum on semis are open (the backing plate does not completely cover them so that the pad thickness can be inspected without taking the system apart). SOP for commercial truck roadside inspection. For disks, all it takes to clear any water off of them is a couple of revolutions (if not just one) with the pads applied. That is NOT true if you get water in drum brakes and there's no easy way for it to get out. And air keeps the brakes off, not on. Run out of air and the system doesn't fade, it locks up, guaranteed. Totally different than (mostly) enclosed hydraulic drum brakes.
Discs cool off WAY faster.