Why is it dangerous to 'fan' the brakes (rapidly pump them) repeatedly when descending a long grade with air brakes?

FANNING (rapidly pumping) the air brakes repeatedly on a long downgrade is dangerous because each application USES compressed air, and rapid repeated pumping can use air FASTER than the compressor can replace it — depleting air pressure and potentially leading to LOSS OF BRAKES (and the spring brakes applying suddenly at low pressure). CDL air brakes. UNLIKE hydraulic brakes (where pumping can help), air brakes should NOT be fanned. PROPER DOWNGRADE TECHNIQUE: select a low gear before descending, use engine braking as the primary control, and use the SNUB method (one firm application to slow ~5 mph below target, then release to let brakes cool and air rebuild, then repeat) — NOT rapid pumping. Knowing not to fan/pump air brakes on downgrades (it depletes air) is commonly tested and safety-critical.