Here’s Why Buses Make That Weird Hissing Sound When They Stop


One of life’s great mysteries has finally been answered thanks to some good old sleuthing on the internet.

The daily commute to work is an arduous one. It involves being rudely awakened by your alarm (in my case I have about seven within the space of an hour), getting physically, mentally and hygienically prepared for the daily grind and then waiting for your mode of transport (almost never ever on time) to bring you to your place of bane work.

For those who use buses as their mode of transport, you may have noticed the hissing sound it makes when it breaks. But why does it make the sound? Do people even care or wonder why?


Well, apparently people do care – unbeknown to me as I’m either listening to music, engrossed in another hilarious Bodega Boys podcast or reading a book about a long-awaited political revolution – I guess the mind needs to be preoccupied somehow.

But the mystery behind one of the most mundane subjects has finally been solved – truth be told the explanation has been around for ages (I suppose people forgot how to google) – and it’s all to do with a bus’s braking system.

Buses use a brake system which releases air from the underside’s holding tank(s).


According to someone only known as RJ on the Yahoo! Answers forum:

Buses have air brakes, and air causes sound when pressurized or when released.

A more detailed explanation on Reddit from mrdotkom explains:

The cool thing about air brakes is that when they fail it’s not catastrophic like when hydraulic brakes fail.

When air brakes fail the brake shoe presses up against the drum because that’s the default state. In fact if you turn a bus on, throw it in gear and try to take off you’ll hear a loud bang and it’ll stall out.

The compressor needs time to fill the system with air to push the brake shoes away from the drum. When you put your foot on the brakes it actually siphons air from the system and presses the shoe against the drum.


This information is also confirmed on Physics Forum, by a ‘Science Advisor’ who goes by the moniker Cliff_J, who writes:

Exhaust braking (a ‘jake brake’) sounds like a loud exhaust system.

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. With brake fluid in a car, air in the system is very bad because of the high pressures and compressibility of air. One air bubble in a fluid brake system could reduce the pressures from 1000psi to 10psi because of this, meaning you’d effectively have almost no brakes.

Removing the air is a process normall called ‘bleeding the brakes’ and this also removes the old fluid. The fluid is hydroscopic, meaning it absorbs water and if the fluid with water would get too warm a steam bubble would form with the same results as air bubbles.

The trucks are also designed with spring backups, so if you would cut the air hose or the system would loose all pressure for some other reason all the brakes come on at full power on the truck to prevent a runaway truck without brakes.


So there you have it when you find yourself rudely awaken on the night bus and there isn’t a person in sight it’s most likely the result of the bus’s air brakes releasing pressure.

Mystery solved.