edit: Solved. Thanks guys
In: Engineering
As most have already said, trucks use air pressure to both remove the parking/secondary brake and to apply the service brake.
A compressor is constantly running which charges several large tanks which in turn provide air to release the parking brake and for service brake applications. When the system reaches a set pressure a Pressure Relief Valve will vent to atmosphere to keep the system pressure as close to constant as possible. This is one occasion where you might hear a release.
The louder releases that you can hear are more than likely as soon as or very shortly after the truck has come to a complete stop. Some of the axles have a Spring Brake Actuator fitted. These use pressure to apply the service brake and to compress a spring. When applying the secondary or parking brake, that pressure is released sharply allowing the spring to reassert itself, applying the brake mechanically. The spring also acts as mechanical fail safe in the event of an air leak. It’s required by law in the UK that all vehicles are fitted with a form of mechanical brake.
In short – apply parking brake, big release of air, gigantic spring applies brakes.
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