Most big trucks have air powered brakes. These are called "air brakes" and they are usually set up in 2 sets. One, the emergency or parking system, is normally

Most big trucks have air powered brakes. These are called "air brakes" and they are usually set up in 2 sets. One, the emergency or parking system, is normally closed rather than normally open. If that system fails, the brakes are immediately applied. The normal operating brakes are Normally open and take pressure from a tank to apply the brake pads. I believe the reason for using air rather than fluid is heat dissipation. Normal fluid brakes would boil if subject to the heat and the fluid would break down. When air brakes get hot, the pressure just goes up.