Air Brakes: Air Brake System Parts

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

Sources: 49 CFR 393.40, 49 CFR 393.45, 49 CFR 393.50, 49 CFR 393.51, 49 CFR 393.55, 49 CFR 396 Appendix A, 49 CFR 571.121

What you need to know

393.51(c)(1) requires a gauge visible from the normal driving position showing the air pressure available for braking, in kPa or psi.

Visible from the normal driving position.

A pressure gauge, visible to a person seated in the normal driving position, which indicates the air pressure (in kilopascals (kPa) or pounds per square inch (psi)) available for braking

49 CFR 393.51

393.51(c)(2) sets 55 psi and below, or half the governor cutout pressure, whichever is the lower figure.

The warning must be continuous, not a single flash or beep. Whichever of the two is less.

A warning signal that is audible or visible to a person in the normal driving position and provides a continuous warning to the driver whenever the air pressure in the service reservoir system is at 379 kPa (55 psi) and below, or one-half of the compressor governor cutout pressure, whichever is less.

49 CFR 393.51

393.51(c) requires both a pressure gauge and a warning signal on any CMV with air service brakes, and on any CMV towing one.

The requirement applies regardless of the date of manufacture.

must be equipped with a pressure gauge and a warning signal

49 CFR 393.51

393.50(d) requires a manually operable condensate drain valve on each reservoir.

Automatic valves are allowed only if they can also be worked by hand or a manual means of draining is kept. Water collecting in a reservoir can freeze and disable the brakes.

Each reservoir must have a condensate drain valve that can be manually operated. Automatic condensate drain valves may be used provided (1) they may be operated manually, or (2) a manual means of draining the reservoirs is retained.

49 CFR 393.50

393.50(b) sets the test as a single full service brake application with the engine stopped, after which pressure must not have fallen below 70 percent of what the gauge showed immediately before.

A full application means the pedal to the limit of its travel. Measured over one full application with the engine stopped.

Each motor vehicle using air or vacuum braking must have either reserve capacity, or a reservoir, that would enable the driver to make a full service brake application with the engine stopped without depleting the air pressure or vacuum below 70 percent of that indicated by the air or vacuum gauge immediately before the brake application is made.

49 CFR 393.50

393.50(c) requires check valves or equivalent devices so that a failure upstream cannot bleed the service reservoir.

Their function must be checkable without disconnecting lines, so the check is part of a normal inspection. Testable without disconnecting any line or fitting.

Each service reservoir system on a motor vehicle shall be protected against a loss of air pressure or vacuum due to a failure or leakage in the system between the service reservoir and the source of air pressure or vacuum, by check valves or equivalent devices whose proper functioning can be checked without disconnecting any air or vacuum line, or fitting.

49 CFR 393.50

FMVSS 121 S5.1.6.1(a) requires direct control of at least one front axle and at least one rear axle on single-unit vehicles built on or after 1 March 1998.

Other axles may be indirectly controlled. With at least one independently controlled.

S5.1.6.1(a) Each single-unit vehicle manufactured on or after March 1, 1998, shall be equipped with an antilock brake system that directly controls the wheels of at least one front axle and the wheels of at least one rear axle of the vehicle.

49 CFR 571.121

393.55(c)(1) sets 1 March 1997 for truck tractors.

Other air-braked CMVs, including trailers, follow from 1 March 1998 under 393.55(c)(2).

Each truck tractor manufactured on or after March 1, 1997 (except truck tractors engaged in driveaway-towaway operations), shall be equipped with an antilock brake system

49 CFR 393.55

393.55(e) requires trailers built on or after 1 March 1998 to carry an exterior ABS malfunction indicator lamp.

FMVSS 121 places it on the left side as viewed from the rear, which is why the walk-around includes looking for it.

Each trailer (including a trailer converter dolly) manufactured on or after March 1, 1998, and subject to the requirements of paragraph (c)(2) of this section, shall be equipped with an ABS malfunction indicator lamp

49 CFR 393.55

FMVSS 121 S5.5.1 requires that an ABS fault must not degrade service brake timing.

You lose the anti-lock function and keep normal braking, so your brake normally and get it repaired rather than stopping use of the brakes.

a malfunction that affects the generation or transmission of response or control signals of any part of the antilock system shall not increase the actuation and release times of the service brakes

49 CFR 571.121

FMVSS 121 S5.1.1.1 sets 100 psi or greater for a truck and 85 psi or greater for a bus.

Cut-in is the pressure at which the governor tells the compressor to start pumping again.

The air compressor governor cut-in pressure for each bus shall be 85 p.s.i. or greater. The air compressor governor cut-in pressure for each truck shall be 100 p.s.i. or greater.

49 CFR 571.121

FMVSS 121 S5.1.1 measures the time to raise supply and service reservoirs from 85 to 100 psi at the manufacturer's maximum recommended engine speed.

A compressor that cannot do it in the required time cannot keep up with repeated brake applications.

An air compressor of sufficient capacity to increase air pressure in the supply and service reservoirs from 85 psi to 100 psi when the engine is operating at the vehicle manufacturer's maximum recommended r.p.m.

49 CFR 571.121

FMVSS 121 S5.1.2.1 requires at least 12 times.

That reserve is what lets you make several full applications after the engine stops, which is why a low air warning is a reason to stop soon rather than instantly. None of the figures offered is the one the rule gives.

The combined volume of all service reservoirs and supply reservoirs shall be at least 12 times the combined volume of all service brake chambers.

49 CFR 571.121

FMVSS 121 S5.2.1.1 requires at least 8 times for a trailer reservoir, against 12 times for a power unit under S5.1.2.1.

The trailer figure is lower because the trailer is resupplied from the towing vehicle.

The total volume of each service reservoir shall be at least eight times the combined volume of all service brake chambers serviced by that reservoir.

49 CFR 571.121

FMVSS 121 S5.1.2.2 requires the greater of five times cutout or 500 psi, held for 10 minutes.

The large margin exists because a reservoir failure would release all stored air at once. Held for 10 minutes.

Each reservoir shall be capable of withstanding an internal hydrostatic pressure of five times the compressor cutout pressure or 500 psi, whichever is greater, for 10 minutes.

49 CFR 571.121

FMVSS 121 S5.1.5 sets below 60 psi for vehicles subject to the standard.

Note this differs from the 55 psi figure in 393.51(c)(2), which applies only to power units FMVSS 121 did not cover. The signal must also be something other than the gauge itself. None of the figures offered is the one the rule gives.

A signal, other than a pressure gauge, that gives a continuous warning to a person in the normal driving position when the ignition is in the “on” (“run”) position and the air pressure in the service reservoir system is below 60 psi.

49 CFR 571.121

They are not in conflict.

FMVSS 121 S5.1.5 governs vehicles built subject to the standard. 393.51(c)(2) sets 55 psi, or half the governor cutout if lower, for power units that were outside FMVSS 121 when built. A question that quotes one figure must be read against the vehicle population it covers. The brake standard applies to vehicles subject to it; 393.51(c)(2) covers power units it did not reach.

A signal, other than a pressure gauge, that gives a continuous warning to a person in the normal driving position when the ignition is in the “on” (“run”) position and the air pressure in the service reservoir system is below 60 psi.

49 CFR 571.121, 49 CFR 393.51

FMVSS 121 S5.1.5 allows a visible signal only if it is within the driver's forward field of view; otherwise it must be both audible and visible.

A warning the driver cannot see from the driving position does not count. Visible alone must be within the driver's forward field of view.

The signal shall be either visible within the driver's forward field of view, or both audible and visible.

49 CFR 571.121

FMVSS 121 S5.1.4 allows plus or minus 7 percent of compressor cut-out pressure.

That tolerance is one reason you act on a low air warning rather than reading the needle precisely.

The accuracy of the gauge shall be within plus or minus 7 percent of the compressor cut-out pressure.

49 CFR 571.121

FMVSS 121 S5.1.7 requires the stop lamp switch to light the lamps at 6 psi or less in the chambers.

The threshold is deliberately low so following drivers see the lamps at the very start of a brake application.

A switch that lights the stop lamps when the service brake control is statically depressed to a point that produces a pressure of 6 psi or less in the service brake chambers.

49 CFR 571.121

FMVSS 121 S5.1.3 states the purpose plainly: keep a trailer problem from becoming a tractor problem.

If a trailer line bursts, the tractor must retain enough air to stop itself. It isolates the tractor's supply so a hose failure behind you do not empty your own reservoirs.

If the vehicle is intended to tow another vehicle equipped with air brakes, a system to protect the air pressure in the towing vehicle from the effects of a loss of air pressure in the towed vehicle.

49 CFR 571.121

FMVSS 121 S5.1.6.3 requires the towing vehicle to supply continuous power to the towed vehicle's ABS whenever the ignition is on.

This is why a damaged trailer cord can disable trailer ABS. Supplied from the towing vehicle.

Each truck tractor manufactured on or after March 1, 1997, and each single unit vehicle manufactured on or after March 1, 1998, that is equipped to tow another air-braked vehicle shall be equipped with one or more electrical circuits that provide continuous power to the antilock system on the towed vehicle or vehicles whenever the ignition (start) switch is in the “on” (“run”) position.

49 CFR 571.121

FMVSS 121 S5.1.8 requires a service brake system acting on all wheels, matching the in-service requirement in 393.42(a).

Each vehicle shall be equipped with a service brake system acting on all wheels.

49 CFR 571.121

FMVSS 121 S5.1.2 allows either an automatic drain on each service reservoir or a supply reservoir placed upstream, which collects water before it reaches the service reservoirs.

That is why the supply or wet tank is drained first. The drain valve is per service reservoir; the supply reservoir sits between the service reservoirs and the air source.

One or more service reservoir systems, from which air is delivered to the brake chambers, and either an automatic condensate drain valve for each service reservoir or a supply reservoir between the service reservoir system and the source of air pressure.

49 CFR 571.121

393.51(d)(2) sets less than 203 mm, which is 8 inches, of mercury.

Vacuum systems are measured in inches of mercury rather than psi because they work below atmospheric pressure. None of the figures offered is the one the rule gives.

A warning signal that is audible or visible to a person in the normal driving position and provides a continuous warning to the driver whenever the vacuum in the vehicle's supply reservoir is less than 203 mm (8 inches) of mercury.

49 CFR 393.51

FMVSS 121 S5.2.1.4 requires check valves or equivalent on the trailer, mirroring S5.1.2.3 on the power unit.

This is why a trailer keeps enough air to set its spring brakes when the supply line fails.

Each service reservoir shall be protected against loss of air pressure due to failure or leakage in the system between the service reservoir and its source of air pressure by check valves or equivalent devices.

49 CFR 571.121

FMVSS 121 S5.2.1 states it plainly: the trailer has reservoirs but no compressor, and the towing vehicle supplies them.

That dependency is why the supply line and the tractor protection valve matter so much.

One or more reservoirs to which the air is delivered from the towing vehicle.

49 CFR 571.121

FMVSS 121 S5.2.2 requires brakes on all wheels for trailers, the same standard S5.1.8 sets for power units and 393.42(a) sets in service.

Each vehicle shall be equipped with a service brake system acting on all wheels.

49 CFR 571.121

396 Appendix A item 1.d(5) lists all three.

A crimped hose passes no air and a cracked one will fail under pressure, so neither needs to be leaking yet to be a defect.

Air hose cracked, broken or crimped.

49 CFR 396 Appendix A

393.45 covers the whole fluid and air carrying path: tubing, hoses, the assemblies and their end fittings.

It is the plumbing between the reservoirs and the chambers.

Brake tubing and hoses; hose assemblies and end fittings

49 CFR 393.45

393.40(b)(3) applies the same five sections used for air systems outside FMVSS 121: brakes on all wheels, brakes operative, a single control valve, warning signals and performance.

Motor vehicles equipped with vacuum brake systems must have a service brake system that meets the applicable requirements of §§ 393.42, 393.48, 393.49, 393.51, and 393.52 of this subpart.

49 CFR 393.40

393.40(b)(5) lists 393.42, 393.48, 393.49 and 393.52 for surge brakes, omitting 393.51.

A surge brake is actuated by the trailer pushing on the hitch, so there is no air or vacuum pressure for a gauge or warning signal to report.

Motor vehicles equipped with surge brake systems must have a service brake system that meets the applicable requirements of §§ 393.42, 393.48, 393.49, and 393.52 of this subpart.

49 CFR 393.40

Check yourself

• What must your air pressure gauge indicate and where must it be visible from?

• What is the pressure at which your low air pressure warning must activate?

• What two devices must your air-braked vehicle have, whenever it was built?

• What must the drain valves on your reservoirs do?

• Your vehicle was built before the federal brake standard applied. What reservoir capacity is required?

• How must your service reservoir be protected against a leak between it and the compressor?

• What wheels must the ABS on your single-unit vehicle directly control?

• From what date would your truck tractor have been required to have ABS?

• How can you tell during a walk-around whether a trailer's ABS is working?

• Your ABS warning lamp comes on while driving. What is the state of your service brakes?

• What is the minimum compressor governor cut-in pressure for your truck?

• Across what pressure range is air compressor capacity measured?

• On your power unit, how large must the reservoirs be relative to your brake chambers?

• On your trailer, how large must each service reservoir be relative to the chambers it serves?

• What burst strength must your power unit air reservoir withstand?

• What is the pressure at which your low air warning signal must operate?

• Why do the brake standard and the operating rule give different low air warning pressures, 60 psi and 55 psi?

• How may the low air warning signal be presented to you?

• What accuracy must your air pressure gauge meet?

• At what brake chamber pressure must the stop lamps come on?

• What is the stated purpose of your towing vehicle protection system?

• How is your towed vehicle's ABS powered?

• What is required of your service brake system?

• What two arrangements does the standard permit for handling condensate in the service reservoir system?

• On a vacuum-braked vehicle, at what level must your warning signal operate?

• What protects your trailer's service reservoir from a leak upstream of it?

• Where does the air in your trailer's reservoirs come from?

• What is required of your trailer's service brakes?

• Which air hose conditions are defects?

• What does the rule cover?

• Which sections govern a vacuum brake service system?

• What requirement applies to air and vacuum systems but NOT to surge brakes?

Answers are in the practice test for this topic.