Pre-Trip Inspection: Air Brake Checks

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

Sources: 49 CFR 570.57

What you need to know

2 psi released for a single vehicle, 3 for a combination.

Note this is the RELEASED test. The applied test allows more, and confusing the two is the commonest mistake on this topic.

Air brake pressure shall not drop more than 2 psi in 1 minute for single vehicles or more than 3 psi in 1 minute for combination vehicles, with the engine stopped and service brakes released.

49 CFR 570.57

4 psi applied for a combination, 3 for a single.

The applied figures are each 1 psi higher than the released ones because applying the brakes pressurizes more of the system, giving leaks more places to appear.

With the reservoir(s) fully charged, air pressure shall not drop more than 3 psi in 1 minute for single vehicles or more than 4 psi in 1 minute for combination vehicles, with the engine stopped and service brakes fully applied.

49 CFR 570.57

One extra psi per added unit.

Every trailer brings its own hoses and glad hands, so the standard scales the allowance with the number of places a leak can be rather than holding a triple to a single's figure. Each additional towed vehicle brings its own hoses and glad hands.

There may be an additional 1 psi drop per minute for each additional towed vehicle.

49 CFR 570.57

20 percent.

This checks that you have reserve enough for a stop after the engine quits, which is the situation the reserve exists for.

compressed air reserve shall be sufficient to permit one full service brake application, after the engine is stopped and with the system fully charged, without lowering reservoir pressure more than 20 percent below the initial reading

49 CFR 570.57

60 psi on a vehicle built to the federal brake standard.

Older vehicles outside that standard are held to 50 psi, so you may see either figure quoted; 60 is the one that applies to anything you are likely to drive.

For vehicles manufactured to conform to Federal Motor Vehicle Safety Standard No. 121, the low-pressure indicator shall be activated when air pressure is lowered to an activating level that is not less than 60 psi.

49 CFR 570.57

80 psi floor on cut-in and 135 psi ceiling on cut-out, unless the manufacturer specifies otherwise.

These are outer limits for an inspection, which is why they are wider than the roughly 100 to 125 psi a healthy system actually cycles between.

The governor cut-in pressure shall be not lower than 80 psi, and the cut-out pressure shall be not higher than 135 psi, unless other values are recommended by the vehicle manufacturer.

49 CFR 570.57

45 seconds on a vehicle built from March 1975 onward, and 30 seconds on older ones.

A compressor that is slow to recover leaves you short of air on a long downgrade with repeated applications. None of the figures offered is the one the rule gives.

For vehicles, manufactured on or after March 1, 1975, the time allowed for air pressure buildup shall not exceed 45 seconds.

49 CFR 570.57

Wear and tension both.

A slipping belt still turns the compressor, just not fast enough, so the symptom is slow pressure build rather than no air at all. It must also be tight enough not to slip: a slipping belt still turns the compressor, just too slowly.

The compressor drive belt shall not be badly worn or frayed and belt-tension shall be sufficient to prevent slippage.

49 CFR 570.57

570.57(b) names all four conditions.

Improper support is the one drivers miss: a line that hangs where it can rub is not yet leaking, which is exactly when you can still fix it cheaply.

Air system tubes, hoses and connections shall not be restricted, cracked or improperly supported, and the air hose shall not be abraded.

49 CFR 570.57

Engine stopped, then look.

A running engine masks small leaks by replacing the air as fast as it escapes, and it is loud enough to hide the hiss.

Stop the engine and examine air hoses, tubes and connections visually for conditions specified.

49 CFR 570.57

Coupled.

A trailer's brakes cannot be tested without the tractor that supplies and controls their air, so the inspection is of the combination rather than of the trailer.

Trailer(s) must be coupled to a truck or truck-tractor for the purpose of this inspection, except as noted.

49 CFR 570.57

Check yourself

• Engine off and brakes released, a single vehicle may lose no more than ______.

• Engine off and brakes applied, a combination may lose no more than ______.

• You are pulling doubles. How does the allowable leakage change?

• What is the maximum pressure drop from one full application with the engine stopped?

• What is the pressure at which your low air warning must activate on a modern vehicle?

• What are the governor cut-in and cut-out limits in the inspection standard?

• What is the maximum time for your compressor to rebuild pressure after a full application?

• How do you check the compressor drive belt?

• What is an air line defect on your walk-around?

• What is the method the standard gives for inspecting air hoses and connections?

• What is the required presentation of a trailer for this brake inspection?

Answers are in the practice test for this topic.