They're actually rubber bladders--god knows maybe they're goatskin--that force the drum-brake shoes out against the drums, and of course they're prone to leakage. The air compressor on an Ant is as important as the APU on a jet.
As I remember--it's been awhile--the brake bladders are activated by a bicycle-grip squeeze handle on the yoke, plus differential rudder-pedal use. Makes a wonderful noise as you taxi in, though.
The other thing I remember about flying the AN-2 is that the windshield-panels configuration is such that there are areas where the wind whips the rain away as you fly through it, and other areas where it just drips and falls pretty much as it does out of a clogged gutter on your porch. Very strange.