Why is air hissing when I release my brake pedal?

The hissing sound you hear when releasing the brake pedal is likely due to the release of compressed air from the brake system, particularly in vehicles equipped with pneumatic brakes, such as trucks or buses. This sound can occur as the brake components retract and air escapes from the brake chambers. Additionally, it may indicate that the brake system is functioning normally, but if the noise is excessive or accompanied by other issues, it could signal a potential air leak or malfunction that should be checked by a mechanic.