Buses, trucks and trailers do make that hissing sound when they stop. I guess it is air, because of those big cylinders at spce between the back of the head and the body. Also the dust that got blown away from under the vehicle.

What is the use of the air?

That’s break relief ‘FALCON’ for any heavy duty vehicles, the brake system mostly works with air filled falcons unlike the regular smaller vehicles

That’s why it’s almost impossible for an heavy duty vehicles to stop immediately the brake is applied when on motion coz the system has to pump some air into the falcon first ……FEW SECONDS THOUGH