Does a air brake endorsement expire on a class B CDL?

No. There is no air brake endorsement. If you completed the written air brakes test and did your road test in a vehicle equipped with air brakes, you're certified to operate an air brake equipped CMV. Otherwise, you'll have a restriction on your license which prohibits you from operating a CMV equipped with air brakes (although this does not apply to non-CMVs, such as U-Haul trucks and RVs).