What is the foot valve (treadle valve) in an air brake system?

The FOOT VALVE (also called the treadle valve or brake pedal) is the mechanism the driver operates with their foot to apply the SERVICE BRAKES. CDL air brakes. Pressing the foot valve releases compressed air to the brake chambers — the harder you press, the more air pressure (application pressure) is delivered and the more braking force is produced; releasing it lets the air out and releases the brakes. The foot valve controls normal service braking. OTHER CONTROLS: the parking brake control (knob for spring brakes); the trailer hand valve/trolley valve (applies only trailer brakes — for testing, not parking). Knowing the foot valve is the service brake pedal that controls application pressure is commonly tested air brake system knowledge.