Have you ever wondered why trucks and buses make those funny squeaking and hissing sounds? The squeaking is the air escaping after braking and the ppssss sound is the automatic bypass safety valves at work, ensuring the air pressure remains at the correct level.

Why do trucks make a hissing sound?

The hissing sound is because instead of fluid like a car would use, the truck uses compressed air to actuate the brake pads. They use very large diaphrams because the pressures are lower. When the truck is hooked up a trailer or should it be disconnected all that comes out of the hose is air.

Why do buses have air brakes?

Design and function. Air brakes are typically used on heavy trucks and buses. ... Setting of the parking/emergency brake releases the pressurized air in the lines between the compressed air storage tank and the brakes, thus allowing the spring actuated parking brake to engage.