ELI5 : why do buses make that “PFSSSST” sound when they stop?

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ELI5 : why do buses make that “PFSSSST” sound when they stop?

In: Engineering

Many large vehicles have a system of brakes actuated by compressed air instead of hydraulic fluid or cables you would find in a lighter vehicle or passenger car.

Parking/emergency brakes on such vehicles are usually “normally closed”: to release the brake, air pressure must be applied to the brake. For these, a sudden release of air pressure will quickly activate that brake, but also will create a little bit of noise.

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Air brakes.

A tank of compressed air hold the brakes off, pressing the brake pedal relieves some pressure allowing the brakes to apply.

The tank is pressurised by a pump on the engine.

When stopped, the parking brake will allow the brakes to fully apply by releasing all pressure in the lines.

It’s far more complicated I’m sure!

Edit: one reason they are used over hydraulic systems is fail-safe and maintenance.
If something failed, the brakes will lock on, not dump fluid under pressure and facilitates road-side repair.