Posted: January 14, 2011

We’ve heard the noise all our lives, but did you ever stop to wonder why buses and big rigs are prone to making those unmistakable hissing sounds?  It’s all about their air brakes.

The hydraulic braking systems generally found in passenger cars and  trucks use pistons to squeeze the brake discs and stop the wheels. But commercial rigs, buses and even rail cars use air brake systems to bring things to a halt.  These rides are all equipped with air reservoir tanks that retained pressurized or compressed air.  When the brake pedal is pushed down, air rushes into the chamber and mechanisms take place that push the brake shoe against the brake drum.  It is this friction and pressure that slows the vehicle.  So … about that hissing noise.  It’s the sound of air escaping after the braking process is completed or the reservoir has too much air.  After that, the compressor refills and pressurizes the air, so that the vehicle is ready for its next rolling stop.