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DC CDL General Knowledge study materials
DC CDL General Knowledge study resources

DC CDL General Knowledge: Vehicle Systems and Pre-Trip Inspection

DC CDL General Knowledge · Work through your exam, one topic at a time.

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Vehicle Systems and Pre-Trip Inspection

This domain tests the numbers and procedures of the inspection itself: air brake tests, pushrod travel, tire tread depth, steering play, and the coupling checks on a combination vehicle. Expect questions that ask you to compare a measurement against a limit.

Air brake tests

  • Build rate: with a dual air system, pressure should rise from 85 to 100 psi within 45 seconds.
  • Static leak test: engine off, full pressure, brakes released, wait one minute - loss should not exceed 3 psi in a single vehicle or 4 psi in a combination.
  • Applied leak test: with the brakes fully applied, loss should not exceed 2 psi single or 3 psi combination in one minute.
  • Low-pressure warning must activate before pressure falls below 60 psi; the spring brakes should set between 20 and 45 psi.

Brake adjustment and running gear

  • With the brakes applied at 90-100 psi, pushrod travel on a standard chamber should not exceed about 2 inches (2.5 inches for long-stroke chambers).
  • Steering play beyond 10 degrees, or about 2 inches at a 20-inch wheel, is a defect.
  • Tire tread: at least 4/32 inch on every front tire and 2/32 inch on all others; no recaps on the steering axle; check for cuts, bulges, and mismatched inflation.

Combination and lights

  • Fifth wheel: jaws locked around the kingpin, no gap, safety latch engaged.
  • Check the trailer emergency air lines and electrical cord for chafing, and test the trailer brakes before driving.
  • Confirm all lights, reflectors, and mud flaps; look for fresh fluid puddles under the engine, transmission, and rear axle.

Worked example

A combination vehicle at full pressure, engine off, brakes released, loses 6 psi in one minute. Reasoning: the limit for a combination is 4 psi, so this fails. The driver records the leak, does not dispatch the vehicle, and reports it for repair rather than repeating the test hoping for a better number.

Traps

  • Running the static test with the brakes applied.
  • Confusing the 2/3 psi applied limits with the 3/4 psi released limits.
  • Assuming self-adjusting slack adjusters mean no measurement.