Brake Pedal Operation, Application, and Release
Pedal feel is a test topic because it explains both how much braking you get and why air brakes feel different from hydraulic brakes.
What the pedal does
The foot valve is a pressure regulator, not a pump. How far you push the treadle sets how much reservoir pressure reaches the chambers; push further and you get more force. Push all the way for a full application. Steady pressure holds a constant application - you do not need to pump or fan the pedal, and doing so wastes air.
Release
Letting up closes the valve and opens an exhaust port; air leaves the chambers (helped by relay and quick-release valves) and return springs pull the shoes off. In a dual-circuit system the foot valve applies both circuits; if one fails you still have the other, but with reduced braking - stop and get it fixed.
Brake lag and holding
Because air must travel, the brakes apply a moment after your foot moves; at 55 mph that lag covers about 32 feet. For a long wait at a light or a railroad crossing, set the parking brake rather than holding the treadle.
Worked example
On a long downgrade your speed creeps up while you hold light pedal pressure. Reasoning: light pressure cannot absorb the heat. Select a lower gear before the grade, then use firm, steady applications and let the engine do most of the work.
Traps
Fanning the pedal to build up braking; believing release dumps the whole system's air (it only exhausts the chambers); using the trailer hand valve to hold the rig at a stop.