Pre-Trip Inspection: Vacuum Brakes

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

Sources: 49 CFR 570.56

What you need to know

8 inches of mercury.

Vacuum systems are measured in inches of mercury rather than psi, which is why a figure in psi is always wrong for this question. None of the figures offered is the one the rule gives.

If the vehicle has a low-vacuum indicator, the indicator activation level shall not be less than 8 inches of mercury.

49 CFR 570.56

One application in reserve.

That is enough to bring the vehicle to a stop after the engine quits, which is exactly the failure the reserve is sized for. It must do that without actuating the low vacuum warning.

The vacuum brake system shall provide vacuum reserve to permit one service brake application with a brake pedal force of 50 pounds after the engine is turned off without actuating the low vacuum indicator.

49 CFR 570.56

It sinks, because the assist has just begun helping you push.

A pedal that does not move when the engine starts means you are braking on leg strength alone.

The vacuum brake assist unit shall demonstrate integrity as indicated by a decrease in pedal height when the engine is started and a constant 50-pound force is maintained on the pedal.

49 CFR 570.56

Check yourself

• What is the level at which a low-vacuum indicator must activate?

• What is a vacuum brake system still required to deliver with the engine off?

• What is the brake pedal expected to do when the engine starts, if the assist works?

Answers are in the practice test for this topic.