Air Brakes: Air Brake Inspection

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

Sources: 49 CFR 393.47, 49 CFR 393.48, 49 CFR 393.49, 49 CFR 393.53, 49 CFR 396 Appendix A, 49 CFR 571.121

What you need to know

393.47(b) requires the service and spring brake chambers on each end of an axle to be the same size.

Mismatched chambers produce different brake force side to side, which pulls the vehicle under braking. Mismatched chambers brake the two sides unevenly.

The service brake chambers and spring brake chambers on each end of an axle must be the same size.

49 CFR 393.47

393.47(c) requires equal effective length on each end of an axle.

The slack adjuster is a lever, so a different length means different brake torque for the same air pressure, and the vehicle brakes unevenly.

The effective length of the slack adjuster on each end of an axle must be the same.

49 CFR 393.47

393.47(d)(2) sets 6.4 mm, which is 1/4 inch, measured at the shoe center for drum brakes, or to the wear indicator if the lining is marked.

Air disc brakes on the same axle are 3.2 mm, or 1/8 inch.

An air braked commercial motor vehicle shall not be operated with brake lining/pad thickness less than 6.4 mm ( 1/4 inch) or to the wear indicator if the lining is so marked (measured at the shoe center for drum brakes)

49 CFR 393.47

393.47(d)(1) sets 4.8 mm, which is 3/16 inch, at the shoe center for a continuous strip.

A steering axle shoe with two pads needs 6.4 mm, or 1/4 inch. Steering axle limits matter most because losing front braking costs you steering control under braking.

The brake lining/pad thickness on the steering axle of a truck, truck-tractor or bus shall not be less than 4.8 mm ( 3/16 inch) at the shoe center for a shoe with a continuous strip of lining

49 CFR 393.47

393.47(e)(2) uses 80 percent of the manufacturer's rated stroke, or the marked readjustment limit.

Beyond that the chamber is running out of travel and the brake cannot develop full force in an emergency. Both figures are marked on the actuator by its manufacturer.

For actuator types not listed in these tables, the pushrod stroke must not be greater than 80 percent of the rated stroke marked on the actuator by the actuator manufacturer, or greater than the readjustment limit marked on the actuator by the actuator manufacturer.

49 CFR 393.47

393.48(a) requires every brake the vehicle has to be capable of operating at all times, subject to narrow exceptions such as a raised lift axle or a towed vehicle with disabling damage.

Cutting out one brake to stop it dragging is a violation.

all brakes with which a motor vehicle is equipped must at all times be capable of operating

49 CFR 393.48

393.48(c)(6) allows lift axle brakes to be inoperative while the axle is raised, because the tires are off the ground, but requires them to work whenever the axle is lowered and the tires are on the road carrying load.

The tires must be in contact with the road.

Brakes on lift axles need not be capable of being operated while the lift axle is raised. However, brakes on lift axles must be capable of being applied whenever the lift axle is lowered and the tires contact the roadway.

49 CFR 393.48

393.53(c) requires a brake adjustment indicator on these vehicles, so under-adjustment is visible during a walk-around rather than needing a stroke measurement.

On each commercial motor vehicle manufactured on or after October 20, 1994, and equipped with an air brake system which contains an external automatic adjustment mechanism and an exposed pushrod, the condition of service brake under-adjustment must be displayed by a brake adjustment indicator

49 CFR 393.53

393.48(b)(1) restricts manual limiting valves to pre-March 1975 vehicles and only in adverse conditions.

The belief that cutting front brakes helps on slippery roads is outdated; front brakes contribute a large share of total stopping force. Adverse means wet, snowy or icy roads; the device may not be used routinely.

Manually operated devices to reduce or remove front-wheel braking effort may only be used on buses, trucks, and truck tractors manufactured before March 1, 1975. Such devices must not be used unless the vehicle is being operated under adverse conditions such as wet, snowy, or icy roads.

49 CFR 393.48

393.48(b)(2) caps the reduction at 50 percent of the force available with the device disconnected, and the device must not operate at all above 85 psig application pressure, so full braking is available in an emergency.

Automatic devices must not reduce the front-wheel braking force by more than 50 percent of the braking force available when the automatic device is disconnected

49 CFR 393.48

393.49(a) requires a single application valve to operate all service brakes.

An additional valve for trailer brakes is permitted, but the primary control must work everything at once. Across the vehicle or combination.

one application valve must when activated cause all of the service brakes on the motor vehicle or combination motor vehicle to operate

49 CFR 393.49

FMVSS 121 S5.2.1.3 requires a manually operable condensate drain valve on each reservoir, matching 393.50(d).

Water that is not drained can freeze and block a line or valve.

Each reservoir shall have a condensate drain valve that can be manually operated.

49 CFR 571.121

393.47(a) names fading, which is losing effectiveness when hot, and grabbing, which is applying more abruptly than the driver asked for.

Both make the vehicle unpredictable under braking.

Brake components must be constructed, installed and maintained to prevent excessive fading and grabbing.

49 CFR 393.47

393.47(f) sets 1.6 mm, which is 1/16 inch.

Wedge brakes are checked by scribe mark movement rather than pushrod stroke because they have no external pushrod to measure.

The movement of the scribe mark on the lining shall not exceed 1.6 mm ( 1/16 inch)

49 CFR 393.47

393.47(g) defers to the manufacturer's limits rather than setting one number, because drums and rotors vary in design.

The limit is normally cast or stamped into the part.

The thickness of the drums or rotors shall not be less than the limits established by the brake drum or rotor manufacturer.

49 CFR 393.47

393.48(b)(2) requires the device to be inoperable above 85 psig on an air-braked vehicle.

Above that pressure the driver is braking hard, and full front braking must be available.

The device must not be operable when the brake control application pressure exceeds 85 psig (for vehicles equipped with air brakes)

49 CFR 393.48

393.53(b) sets 20 October 1994 for air brake systems.

Hydraulic systems came a year earlier, on 20 October 1993, under 393.53(a).

Each commercial motor vehicle manufactured on or after October 20, 1994, and equipped with an air brake system must meet the automatic brake adjustment system requirements of Federal Motor Vehicle Safety Standard No. 121

49 CFR 393.53

393.47(a) names both faults.

Fading is losing effectiveness when hot; grabbing is applying more sharply than commanded. Grabbing is the more immediately dangerous because it can lock a wheel without warning. A grab, rather than a failure to apply.

Brake components must be constructed, installed and maintained to prevent excessive fading and grabbing.

49 CFR 393.47

396 Appendix A item 1.f fails the device on any of three grounds.

Note the 55 psi figure matches 393.51(c)(2), so the inspection standard and the equipment rule line up. Any of the three is a defect. None of the figures offered is the one the rule gives.

Low Pressure Warning Device missing, inoperative, or does not operate at 55 psi and below, or

49 CFR 396 Appendix A

396 Appendix A item 1.g fails it for being inoperable as well as missing.

A valve that is present but does not work provides no protection at all.

Inoperable or missing tractor protection valve(s) on power unit.

49 CFR 396 Appendix A

396 Appendix A item 1.a(4) treats an audible leak at the chamber as a defect and names the usual causes.

A ruptured diaphragm means that chamber is no longer producing braking force.

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

49 CFR 396 Appendix A

396 Appendix A item 1.a(1) starts with the most fundamental failure: an axle that should brake and does not, whether because brakes are missing or because the shoes do not move when applied.

Absence of braking action on any axle required to have brakes upon application of the service brakes (such as missing brakes or brake shoe(s) failing to move upon application of a wedge, S-cam, cam, or disc brake).

49 CFR 396 Appendix A

396 Appendix A item 1.j(4) is explicit that manual adjustment without fixing the cause is not acceptable.

An automatic adjuster that is not adjusting is usually a symptom of something else, such as broken welds, loose mounting hardware or a cracked drum. Manually adjusting it is not acceptable on its own: it hides the fault rather than fixing it.

Any brake that is found to be out of adjustment on initial inspection must be evaluated to determine why the automatic brake adjuster is not functioning properly and the problem must be corrected in order for the vehicle to pass the inspection. It is not acceptable to manually adjust automatic brake adjusters without first correcting the underlying problem.

49 CFR 396 Appendix A

396 Appendix A item 1.d(2) fails a drum or rotor with any portion missing or about to detach.

A piece leaving a spinning drum is a projectile as well as a brake failure.

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

49 CFR 396 Appendix A

396 Appendix A lists all four, and heat damage is worth noting: tubing routed near an exhaust component can be cooked without any visible leak until it fails under pressure.

Tubing cracked, damaged by heat, broken or crimped.

49 CFR 396 Appendix A

396 Appendix A lists a cracked, broken or loose pulley among the air compressor defects.

Lose the pulley and the compressor stops turning, so the system stops making air entirely.

Cracked, broken or loose pulley.

49 CFR 396 Appendix A

396 Appendix A ends the ABS items with a catch-all, so a component not specifically named is still a defect if it is missing or inoperative.

Other missing or inoperative ABS components.

49 CFR 396 Appendix A

Check yourself

• What is required of the brake chambers on a single axle of your vehicle?

• What is required of the slack adjusters on a single axle of your vehicle?

• You measure lining on a non-steering axle with air drum brakes. What is the minimum thickness?

• On a steering axle with a continuous strip of lining, the minimum thickness is ______.

• Your actuator type is not in the pushrod stroke tables. What limit applies?

• What is the general rule in the rule about the brakes a vehicle is fitted with?

• What is required of the brakes on your lift axle?

• On a post-1994 air-braked vehicle with an external automatic adjuster and exposed pushrod, how is under-adjustment shown?

• When may you use a manual front-wheel brake limiting device?

• By how much may an automatic front-wheel limiting device cut your front braking force?

• What is required of your service brake control on a power-braked vehicle?

• What does the standard require on each trailer reservoir for draining?

• What two behaviors does the rule require brake components to be built and maintained to prevent?

• On a wedge brake, what is the maximum permitted movement of the scribe mark on your lining?

• What sets the minimum drum or rotor thickness?

• Above what application pressure must an automatic front-wheel brake limiting device stop operating?

• From what date must air-braked CMVs have automatic brake adjusters?

• What is brake grabbing, as distinct from fading?

• When does a low pressure warning device fail the periodic inspection?

• What makes your tractor protection valve a defect at periodic inspection?

• What is indicated by an audible air leak at your brake chamber?

• What is the first service brake defect listed in the periodic inspection standards?

• An automatic slack adjuster is found out of adjustment. What does the inspection standard require?

• What drum or rotor condition is a defect?

• Which brake tubing conditions are listed defects?

• What compressor-related defect does the inspection standard list?

• How does the inspection standard close out its ABS defect list?

Answers are in the practice test for this topic.