system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If hydraulics are used because liquids aren't compressible why do they use pneumatic brakes in large vehicles like buses and lorries?
gpt
There are two primary reasons. One applies more to trucks, the other to passenger buses.

1) - Trucks - most trucks have multiple, interchangeable components (cabs and trailers). Each component has it's own braking system. These systems all have to be interconnected so they can be controlled from one location (the brake pedal in the cab). This means frequently connecting/disconnecting brake lines. Disconnecting lines in a hydraulic system is a pain. Fluid leaks and has to be replaced, then pressure has to be restored to the system. In a pneumatic system, however, it's easy. Only air leaks out, so no mess. And the system is pressurized off a compressor attached to the engine, so that's primed as soon as you start her up.

Basically, think about a pool vacuum vs. a house vacuum. You can easily add/remove attachments from a home vacuum, all the tubes are filled with air. Try changing out a pool filter hose without getting water everywhere. Plus, if too much leaks out, you might have to re-prime the filter pump.

2) - Buses - Hydraulic brake systems are forced closed by pressure. What this means is when you push the brake pedal, you add pressure, which engages the brake. Pneumatic brakes are forced *open* by pressure. While the vehicle is running the system is pressurized, holding the brakes open. When the pedal is pressed, a valve is opened, the pressure is released, and the brakes swing closed.

Basically what this means is when a hydraulic system fails and loses pressure, the brakes are stuck open, and the vehicle can't stop. When a pneumatic system fails, the brakes are stuck closed, and a vehicle can't go. Either system is fine, because you always have an e-brake... but when it comes to a bus carrying 30 children to and from school everyday, a lot of people like the idea of a bus that won't even move if the break system is faulty.