Actually any vehicle today with air brakes (including school buses) use air pressure to apply the brakes. Increase in air pressure increases the braking.
All air brake vehicles
also have parking/emergency brakes that are spring powered and requires air pressure to release the brake. Thus any air loss in the system, intentional or not, causes the brakes to apply.
Trailers with air brakes have two air lines attached to the tractor. One is a live air line and may or may not have an air tank on the trailer. The second line is a signal line that actuates the trailer air valve and brakes in unision with the tractor.