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A
Brakes wearing out from normal use over thousands of miles
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B
A temporary reduction in braking force caused by overheating brake drums or discs, typically from riding the brakes on long descents
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C
Brakes that don't make contact with the drum
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D
Air leaking from the brake chambers
Why this is the answer
BRAKE FADE is the reduction in braking effectiveness caused by overheating brake components. HOW IT HAPPENS: When a brake pad or shoe presses against a drum or rotor, friction converts kinetic energy to heat. If brakes are applied continuously for too long (as on a long mountain descent), the drum/rotor and friction material absorb more heat than can be dissipated to the air — the surface temperature rises to 400-600°F+. At these temperatures, the friction material (brake lining) undergoes chemical changes that reduce its coefficient of friction — it literally loses the ability to grip. FADE INDICATORS: Increasing pedal travel needed for same stopping force; brakes feeling 'soft' or 'spongy'; burning smell; reduced effectiveness in emergency stops. TWO TYPES: (1) THERMAL FADE — hot friction material loses gripping ability; occurs from sustained application as described; (2) VAPOR LOCK (for hydraulic brakes) — brake fluid boils, gas bubbles compress instead of transmitting pressure. PREVENTION: Use engine braking (downshift on grades) instead of riding friction brakes; use the stab braking technique if brakes are hot (press hard, release, press again); know where escape ramps are. RECOVERY: Stop and let brakes cool (15-30 minutes); do NOT continue driving with faded brakes — they won't recover while still hot and being used. This is why FMCSA regulations require CMV drivers to test brakes before long descents.
Source: FMCSA CDL Manual, Chapter 7, Mountain Driving, Brake Fade