Air
Brakes

Air brakes are really three systems sharing parts: the service brake (normal pedal braking), the parking brake, and the emergency brake (kicks in if the system fails).

• Air compressor pumps air into the storage tanks; the governor cuts the compressor out around 125 psi and cuts it back in around 100 psi.
• Air tanks hold enough air for several brake applications even if the compressor quits. Drain them daily (manually, unless your tanks are automatic) — trapped water can freeze and cause brake failure.
• Safety relief valve on the first tank opens around 150 psi if pressure gets dangerously high — if it ever releases air, that's a sign something's wrong and needs a mechanic.
• S-cam drum brakes are the most common foundation brake — air pressure twists the camshaft, which forces the shoes against the drum. Wedge brakes and disc brakes exist but are less common.
• Low air pressure warning (light and/or buzzer) must activate before tank pressure drops below 55 psi (large buses often warn at 80–85 psi).
• Spring brakes are the mechanical parking/emergency brake — held off by air pressure, they snap on automatically if pressure drops into the 20–45 psi range (commonly 20–30 psi). Don't wait for that to happen — stop safely the moment the warning comes on.
• Never push the brake pedal down while the spring brakes are set — combined spring + air force can damage the brakes.
• ABS: required on air-brake vehicles built on/after March 1, 1998. A yellow malfunction lamp on the dash (or trailer's left front/rear corner) tells you if it's working.

Most heavy vehicles run a dual air system — a "primary" circuit (usually rear brakes) and a "secondary" circuit (usually front), each with its own tanks and lines, both driven by the same pedal.

• Before driving, let the compressor build at least 100 psi in both systems and watch both gauges.
• The warning light/buzzer should clear once both systems rise above the manufacturer's setting (always more than 55 psi), and should trigger again before either system drops below 55 psi.
• If one system loses pressure, only half your brakes work fully — stopping takes longer. Stop safely right away and get it fixed.

• Slack adjusters: park, chock wheels, release the parking brake, and pull hard on each one you can reach. If it moves more than about 1 inch at the push rod, it likely needs adjustment. Vehicles since 1994 have automatic slack adjusters, but still check them — manually adjusting one is only a temporary fix and usually means something underlying needs a mechanic.
• Brake drums/discs: no cracks longer than half the friction area's width; linings can't be loose, soaked in oil/grease, or dangerously thin; hoses can't be cut or worn.

• Air brakes have lag — about a half-second or more for air to reach the brakes after you push the pedal, adding roughly 32 feet at 55 mph on top of perception/reaction/braking distance. Total stopping distance at 55 mph can exceed 450 feet.
• Controlled braking: apply as hard as possible without locking the wheels, with minimal steering input; release and reapply if you need a bigger steering correction or the wheels lock.
• Stab braking: brake fully, release the instant the wheels lock, then reapply fully as soon as they start rolling again — it can take up to a second for wheels to start rolling, so don't reapply too early or the vehicle won't straighten out.
• Downgrades: get into the right low gear before starting down — braking is only meant to supplement engine braking, not replace it. Apply firmly enough to feel a real slowdown, hold until about 5 mph under your "safe" speed, release for a few seconds, then repeat.
• Brake fade comes from overheating — too much brake use without relying on engine braking. Out-of-adjustment brakes overheat faster and fade sooner than properly adjusted ones.
• Don't set the parking brake if the brakes are very hot or very wet in freezing conditions — heat can damage hot brakes, and wet brakes can freeze solid. Use wheel chocks instead and let them cool/dry first.