Pre-Trip Inspection: Brakes

6 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 396 Appendix A

What you need to know

Engine off, 80 to 90 psi, fully applied.

Measuring at a different pressure gives a different stroke, so the conditions are part of the standard rather than detail around it. Measuring at another pressure gives a different stroke.

Stroke must be measured with engine off and reservoir pressure of 80 to 90 psi with brakes fully applied.

49 CFR 396 Appendix A

All three.

The audible leak is the one you can find without tools: with the engine off and the system charged, you listen at each wheel.

Loose brake components including air chambers, spiders, and cam shaft support brackets.

Audible air leak at brake chamber (Example-ruptured diaphragm, loose chamber clamp, etc.).

Missing brake on any axle required to have brakes.

49 CFR 396 Appendix A

Unequal force at the two front wheels steers the vehicle during braking, exactly when you least want an input you did not make.

Slack adjuster length has the same effect. Different chamber sizes brake the two front wheels unevenly.

Mismatch across any power unit steering axle of: (a) Air chamber sizes. (b) Slack adjuster length.

49 CFR 396 Appendix A

Any part missing OR about to come away.

A fragment leaving a drum at road speed is a projectile, quite apart from the braking you lose.

Any portion of the drum or rotor missing or in danger of falling away.

49 CFR 396 Appendix A

The test is whether it OPENS when the brake is applied.

Heat check is a fine surface craze from normal thermal cycling; a flexural crack moves under load and is on its way through. A short hairline heat check is normal thermal craze and does not count.

With any external crack or cracks that open upon brake application (do not confuse short hairline heat check cracks with flexural cracks).

49 CFR 396 Appendix A

Through the reinforcement ply, which is the layer carrying the pressure.

Damage to the cover alone is not the defect, but it is where you start looking.

Hose with any damage extending through the outer reinforcement ply.

49 CFR 396 Appendix A

Check yourself

• What are the conditions under which pushrod stroke is measured?

• What is your brake system out-of-service condition?

• Why do air chamber sizes have to match across a steering axle?

• What is the drum or rotor condition that is an out-of-service defect?

• What drum cracks matter, and which do not?

• What air hose damage puts your vehicle out of service?

Answers are in the practice test for this topic.