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Air Brake System Balance

The air brake system is one of the most important

safety systems on a vehicle. Air brake systems

are carefully designed with valves, tubing or hoses,

etc., selected to result in balanced vehicle braking

performance - one where all the brakes apply as close

to simultaneously as possible and with the desired

amount of force. Proper vehicle maintenance will

result in the original performance being retained, and

so component replacement and general brake system

maintenance are very important. Routine system

maintenance operations should be performed with

this in mind.

Two major areas that we cover here are:

I. Pneumatic (Air) and II. Mechanical.

I. Pneumatic (Air) Systems

General

An ideal or balanced braking system is one in which

the braking pressure reaches each actuator as

close to simultaneously as possible and at the same

pressure level (and at an appropriate level to achieve

torque balance with respect to the axle’s loading). We

recommend downloading the BW1555 Air Pressure

Balance & Threshold Pressure Tests worksheet from

the Literature Center at www.bendix.com.

Transmission Time

Vehicle manufacturers must comply with the air system

timing requirements of Government regulations

(e.g. U.S. FMVSS 121, Canadian CMVSS 121). In

establishing this performance, vehicle manufacturers

carefully select tubing and hose sizes. Air application

and release performance is partially dependent upon

the size and volume of chambers, vehicle weights and

locations of the valves and chambers, or distance the

air must travel. Performance is engineered into the

vehicle by the manufacturer; the role of the vehicle

owner and/or mechanic is to preserve that pneumatic

performance. Here are a few tips to assist in that effort.

Tubing

When replacing tubing or hoses, always replace

with DOT or SAE approved tubing of the same size.

These sizes have been determined by the vehicle

manufacturer to obtain desired performance. If copper

tubing is used, always make sure to ream and de-burr

the tubing after cutting. Check carefully for restrictions

such as kinking or bending, and make sure tubing and

hoses are properly supported. Use the proper size

ﬁ ttings and make certain they are not restricted. As a

rule, do not replace straight ﬁ ttings with elbows. Note:

it takes as much time for air to ﬂ ow through an elbow

ﬁ tting as through 6-7 feet of tubing or hose.

Valving

When replacing valves in the air brake system, be

sure that the function of the replacement valve is

comparable with the valve being replaced. Wherever

possible, we recommend genuine Bendix parts be

used. Then you can ensure that the new valve is

equal in performance to the original. Bendix adheres

to an industry recommendation which requires that,

for some valves, the valve type and some of its critical

characteristics are identiﬁ ed by a metal tag or washer

to help select the correct replacement. (For example:

relay valve crack pressures.)

Air System Contamination

Air brake systems need clean air to function at their

best. Almost all vehicles with air brakes now use an

air dryer to remove contaminants from the air before

it enters the system.

A properly maintained desiccant-type air dryer will

remove almost all the water vapor and oil droplets

present in the compressed air arriving from the air

compressor.

Typically, air dryer cartridge replacement intervals are:

• Every 3 years/300,000 miles for a low air use

vehicle (e.g. line haul, single trailer, with 5 or

less axles) or,

• Every year/100,000 miles for a high air use

vehicle (e.g. refuse truck, transit bus, bulk

unloader, or line haul with 6 or more axles.)

A properly maintained air dryer will substantially reduce

the potential for freeze-ups in the system.

If for some reason an air dryer is not in use, daily

reservoir draining can help reduce the amount of

contamination in the system. While reservoir draining

can not eliminate valve contamination, it can reduce

the amount.

Trailer Air System Contamination

Contamination by insects, fibrous material, etc.

can affect trailer air brake systems. The Cyclone

DuraDrain

™

trailer water separator installed in the

trailer control and/or supply lines near the gladhands

where it self-purges liquid contaminates, contains

solid contaminants and improves the life of the trailer

system components. Other Bendix products that work

to help trailer air system are: the AF-3

™

in-line air ﬁ lter

screens out foreign material from trailer air lines; and

the System-Guard

trailer air dryer which removes

moisture and contaminates from the trailer air system.

Air Brake System Balance: Pneumatic Systems

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BENDIX MC-12 MODULATOR CONTROLLER ASSY Guida alla Risoluzione dei Problemi Pagina 59

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