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Air Brakes: Dual Air Brake Systems

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

Sources: 49 CFR 393.40, 49 CFR 393.41, 49 CFR 393.43, 49 CFR 396 Appendix A, 49 CFR 571.121

What you need to know

393.43(a) requires automatic operation between 20 and 45 psi.

The valve's job is to seal off the tractor's air supply when a trailer breaks away or a line fails, so the tractor keeps enough air to stop itself.

For air braked towing units, the tractor protection valve or similar device shall operate automatically when the air pressure on the towing vehicle is between 138 kPa and 310 kPa (20 psi and 45 psi).

49 CFR 393.43

393.43(b) requires two means.

One operates automatically when the towing vehicle's air supply falls to a fixed pressure between 20 and 45 psi. The other is a manual control the driver can reach from the driving seat.

Every truck or truck tractor equipped with air brakes, when used for towing other vehicles equipped with air brakes, shall be equipped with two means of activating the emergency features of the trailer brakes.

49 CFR 393.43

393.43(b) forbids any arrangement where working the manual control could block the automatic one.

The automatic means is the last line of defense in a breakaway, so nothing the driver does may disable it. Arranging it so that using it prevents automatic operation is prohibited.

The other means shall be a manually controlled device readily operable by a person seated in the driving seat. Its emergency position or method of operation shall be clearly indicated. In no instance may the manual means be so arranged as to permit its use to prevent operation of the automatic means.

49 CFR 393.43

393.43(d) requires automatic and immediate application on breakaway.

This is the whole purpose of the spring brake design: loss of air is what applies them.

Every trailer required to be equipped with brakes shall have brakes which apply automatically and immediately upon breakaway from the towing vehicle.

49 CFR 393.43

393.43(d) sets a minimum of 15 minutes.

A trailer that lost its brakes sooner could begin rolling before anyone reached it.

The brakes must remain in the applied position for at least 15 minutes.

49 CFR 393.43

393.43(e) requires the trailer's own supply reservoir to be protected against backflow.

Without it, a leak on the tractor would drain the trailer's air too, leaving the trailer with nothing to brake with. It matters when the towing vehicle itself loses pressure.

Air brake systems installed on towed vehicles shall be so designed, by the use of “no-bleed-back” relay emergency valves or equivalent devices, that the supply reservoir used to provide air for brakes shall be safeguarded against backflow of air to the towing vehicle upon reduction of the towing vehicle air pressure.

49 CFR 393.43

393.40(b)(2) allows the emergency control to share with the service control OR with the parking control, then closes the door: all three may not be combined.

One of the pairings is fine; collapsing everything into one lever is not.

The emergency brake control may be combined with either the service brake control or the parking brake control. However, all three controls may not be combined.

49 CFR 393.40

393.41 requires holding under any loading found on a public road, with the ice and snow caveat recognizing that no brake holds on a frictionless surface.

'Any condition of loading' means a legally loaded vehicle, not just an empty one. The standard assumes a surface free of ice and snow, and applies to the combination, not just the tractor.

The parking brake shall be capable of holding the vehicle or combination of vehicles stationary under any condition of loading in which it is found on a public road (free of ice and snow).

49 CFR 393.41

393.41(c)(2) requires muscular effort or spring action.

If any other energy applies it, that energy must be isolated from any common source and reserved exclusively for the parking brake, so a failure elsewhere cannot leave the vehicle unable to park.

The parking brake system shall, at all times, be capable of being applied by either the driver's muscular effort or by spring action. If other energy is used to apply the parking brake, there must be an accumulation of that energy isolated from any common source and used exclusively for the operation of the parking brake.

49 CFR 393.41

393.41(c)(3) requires the holding force to come from something other than fluid, air or electricity, which in practice is a mechanical spring.

If it were held by air, a slow leak overnight would release the brake and the vehicle would roll away.

The parking brake system shall be held in the applied position by energy other than fluid pressure, air pressure, or electric energy. The parking brake system shall not be capable of being released unless adequate energy is available to immediately reapply the parking brake with the required effectiveness.

49 CFR 393.41

393.41(b) exempts agricultural commodity, heavy hauler and pulpwood trailers from the FMVSS 121 parking brake requirement, but requires them to carry enough chocking blocks to prevent movement when parked.

An agricultural commodity trailer, heavy hauler or pulpwood trailer shall carry sufficient chocking blocks to prevent movement when parked.

49 CFR 393.41

393.40(d)(2) sets 1 March 1975 for buses, trucks and truck tractors, and 1 January 1975 for trailers.

Trailers came first by two months.

Buses, trucks and truck tractors manufactured on or after March 1, 1975, and trailers manufactured on or after January 1, 1975, must be equipped with an emergency brake system which, at a minumum, meets the requirements of FMVSS No. 121

49 CFR 393.40

393.41(b) names four exclusions: agricultural commodity trailers, converter dollies, heavy hauler trailers and pulpwood trailers.

Three of them must instead carry chocking blocks sufficient to prevent movement when parked.

Each air-braked bus, truck and truck tractor manufactured on and after March 1, 1975, and each air-braked trailer except an agricultural commodity trailer, converter dolly, heavy hauler trailer or pulpwood trailer, shall be equipped with a parking brake system as required by FMVSS No. 121

49 CFR 393.41

393.43(b) requires the automatic means to trigger at a fixed pressure no lower than 20 psi and no higher than 45 psi, the same band as the tractor protection valve in 393.43(a).

One of these means shall operate automatically in the event of reduction of the towing vehicle air supply to a fixed pressure which shall not be lower than 20 pounds per square inch nor higher than 45 pounds per square inch.

49 CFR 393.43

FMVSS 121 S5.6.2 specifies a 20 percent grade on smooth dry portland cement concrete, in both directions.

Holding downhill as well as uphill matters because the brake must resist the vehicle rolling either way. None of the figures offered is the one the rule gives.

With all parking brakes applied, the vehicle shall remain stationary facing uphill and facing downhill on a smooth, dry portland cement concrete roadway with a 20-percent grade

49 CFR 571.121

FMVSS 121 S5.6.3.2 requires a purely mechanical hold that survives total loss of air and needs no electricity.

This is the spring in a spring brake, and it is why a vehicle left overnight does not roll away as its system leaks down.

A mechanical means shall be provided that, after a parking brake application is made with the pressures in the vehicle's parking brake chambers at the levels determined in S5.6.3.4, and all air and fluid pressures in the vehicle's braking systems are then bled down to zero, and without using electrical power, holds the parking brake application

49 CFR 571.121

FMVSS 121 S5.6.3.1 requires the parking brake to survive any single leakage failure in any other brake system.

The parking brake is the fallback, so it cannot depend on the systems it is backing up. The minimum performance must still be met.

The parking brake system shall be capable of achieving the minimum performance specified either in S5.6.1 or S5.6.2 with any single leakage-type failure, in any other brake system, of a part designed to contain compressed air or brake fluid

49 CFR 571.121

FMVSS 121 S5.6.1 uses a drawbar pull with every other brake disabled, so the figure is the parking brake's own contribution and nothing else's.

The alternative route to compliance is the 20 percent grade hold in S5.6.2. Forward or rearward, so only the brake under test resists.

With all other brakes made inoperative, during a static drawbar pull in a forward or rearward direction, the static retardation force produced by the application of the parking brakes shall be:

49 CFR 571.121

396 Appendix A lists a missing or inoperative check valve as a defect.

Check valves are what 393.50(c) and FMVSS 121 rely on to stop a leak upstream draining the service reservoir.

Missing or inoperative check valve.

49 CFR 396 Appendix A

393.43(c) requires a second manual control independent of brake air, hydraulic and other pressure, unless the system automatically applies the towed vehicle's brakes when that pressure fails.

It need not provide modulated braking. It must be able to operate the towed vehicle's brakes in an emergency.

Every truck tractor and truck when used for towing other vehicles equipped with vacuum brakes, shall have, in addition to the single control required by § 393.49 to operate all brakes of the combination, a second manual control device which can be used to operate the brakes on the towed vehicles in emergencies.

49 CFR 393.43

Check yourself

Answers are in the practice test for this topic.