When the Bus stops the driver flips a switch that relieves some sort of pressure, what does that do?

A bus, as well as tractor trailer trucks use compressed air for their braking system as opposed to hydraulic fluid as to most cars and trucks. This is for safety reasons as these are much larger vehicles and the air compressor on board can keep a constant supply of air in the system should a line develop a leak whereas hydraulic fluid would drain out and render the braking system useless. The hissing sound you hear when the bus stops is the brake system purging off excessive pressure build up in the system.

There are valves throughout the system that "bleed off" extra air when it gets above a certain pressure. This causes the hissing sound.

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