It is hard to make apples to apples comparisons since most wet brakes these days are all disc brakes.
I will grant you that the wet disc brakes are pretty solid and that they are more than a step above the drum brakes they replaced.
But the reality is, when brakes get hot the heat transfers to the brake fluid. If the brake fluid gets hot enough it can boil away to nothing and the only braking that is left is any engine braking from down shifting or pulling back on the hand brake.
Back in the day when all brakes on buses were drum brakes, most of the rear drums on wet brakes were about the width of the front drums on the air brake buses. And the rear brakes on the air brake buses were probably half again as wide as the rear drums on the wet brake buses.
I will grant you that spe'c's can vary greatly. I came across an IC RE bus with air brakes that seemed pretty whimpy. They were much smaller than the minimum spe'c for brakes here in WA and OR. The minimum spe'c on a Type D bus is 16.5"x6 in front and 16.5"x8" in the rear. On a Type 'C' bus 16.5"x5" in front and 16.5"x7" in the rear. The whimpy brakes on that IC RE had 6.5" wide in the rear and 5" wide in the front. That is a significant reduction in braking capacity for nearly identical buses.
The size of the air cans that operate the brakes can be very different as well. Most Type 'D' buses in WA and OR will have 30/30 rear cans and 24 cans in the front.
My Crown had 7" wide brakes in the front and 11" wide brakes in the rear. It had 36/36 cans in the rear 30 cans in the front. We had one Gillig that had 9" front brakes and 13" rear brakes. Let me tell you, when you got used to the brakes on those buses when you got into lesser buses you almost felt like you needed to do a Fred Flintstone to get the lesser buses stopped.
