Troubleshooting guide

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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 you use only genuine

Bendix

parts. With genuine Bendix components

you can be conﬁ dent that the new valve is equal in

performance to the original. Bendix adheres to an

industry recommendation requiring that, for some valves,

the valve type and some of its critical characteristics be

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. In fact, Bendix was the ﬁ rst to design and launch

the ﬁ rst commercial vehicle air dryer.

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 three years/300,000 miles for a low-air-use

vehicle (e.g., line haul, single trailer, with ﬁ ve or fewer

axles)

•Every year/100,000 miles for a high-air-use vehicle (e.g.,

refuse truck, transit bus, bulk unloader, or line haul with

six 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 cannot eliminate valve

contamination, it can reduce the amount.

Alcohol Usage in the Air Brake System

Bendix discourages the use of alcohol in the air brake

system as a means of preventing system freeze-up in

cold temperatures. Studies indicate that using alcohol

and alcohol based products sold for this purpose

removes the lubrication from the components of the

air braking system. In addition, the materials used for

the internal seals of the air system components may

be adversely impacted by the residue that some anti-

freeze additives leave behind. Both are detrimental to air

system component life expectancy, causing premature

wear. Because of this, Bendix

air system components

warranty will be void if analysis shows that alcohol was

added to the air brake system.

Air Brake System Balance: Mechanical Systems

Trailer Air System Contamination

Contamination by insects, ﬁ brous material, etc., can

affect trailer air brake systems. The Bendix

Cyclone

DuraDrain

trailer water separator is installed in the

trailer control and/or supply lines near the gladhands.

The separator self-purges liquid contaminants, contains

solid contaminants, and improves the life of the trailer

system components. Other Bendix products that work to

help the trailer air system are: the AF-3

™

in-line air ﬁ lter,

which screens out foreign material from trailer air lines;

and the System-Guard

trailer air dryer, which removes

moisture and contaminants from the trailer air system.

II. Mechanical Systems

General

Vehicle manufacturers must also comply with

government regulations for braking performance

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

with the air or pneumatic side of the system, the vehicle

manufacturer must carefully design the brake geometry,

size, and power of the foundation brake components to

achieve the desired vehicle performance. Maintenance

personnel must likewise strive to maintain this

performance.

Actuators

Brake chambers convert air pressure into mechanical

force. Always maintain the chambers to their original

performance condition. If chamber return springs need

replacement, the springs should be replaced with springs

of the proper spring load. When replacing, remember the

chamber return spring affects the net force delivered by

the chamber, and is especially important in low service

brake applications. For this reason, always replace the

return spring on both chambers on an axle.

Chamber diaphragm life will vary according to the type of

service and the diaphragm environment. Experience will

generally dictate replacement frequency, and it is good

practice to replace all diaphragms on the vehicle at the

same time.

Also, in the case of S-Cam brakes, make sure the

chamber push rods are in line with the slack adjusters. If

misaligned, the chamber rod can rub on the chamber’s

non-pressure plate and cause a dragging brake.

Another factor that inﬂ uences the brake chamber output

force is the chamber push rod length. Ideally, the push

rod length should be selected so that when the chamber

reaches half its maximum stroke, a 90-degree angle

(approximately) is formed between the slack adjuster

and chamber push rod.