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brake care and adjustment

The trailer brake system will perform safely and

efficiently only as long as it is properly maintained

and not abused. Trailer brakes should be inspected

frequently in connection with a Trailer Preventive

Maintenance (TPM) Program. Out-of-adjustment brakes

can cause increased stopping distance, shorter brake

component life, and a greater tendency for the trailer

to jackknife.

warning

Prior to performing maintenance on any air

brake system component, chock the wheels.

The system air pressure should be exhausted

by opening the reservoir drain cocks.

AIR SYSTEM AND BRAKE OPERATION

Proper operation of the brake systems requires a

firm seal between the air brake couplers. Inspect the

couplers for seal damage and cracked housings. Some

couplers are equipped with filters. These filters must be

cleaned at regular intervals to prevent malfunction of

the brake systems. Inspect the air hoses for cracking

and for frayed connections. Be sure air hoses are not

rubbing on any metal surface or each other. Replace or

repair damaged components.

Keep the air system clean. All air tanks should be drained

daily to remove moisture and other contaminants. See

Freezing Weather Maintenance.

Some air valve manufacturers discourage the use of air

line antifreeze. Use may result in deterioration of seals

in these valves.

If you use Teflon tape or other thread sealers to seal

threaded connections in your air lines, be careful not

to allow pieces of the sealer to enter the air system.

It can clog passages into the valves and cause them

to malfunction.

Keep the air system tight. The air system cannot be

charged properly if there are leaks in reservoirs, lines,

hoses, or valves. Always check the tractor pressure

gauge for unusual drops or extended buildup times.

Run the tractor engine until the air brake system

pressure gauge shows at least 105 psi. Listen for air

leaks. With the engine off, check the gauge reading

with no brakes applied. The gauge reading loss should

not exceed three psi in one minute.

With the engine still off, apply the brakes fully for two

minutes. The gauge reading drop should not exceed

four psi in one minute.

With engine still off, slowly open drain cocks in the

trailer's air tanks and allow the pressure to drop gradually.

In a system employing spring brake control valves, the

spring brakes should function and apply the brakes.

In a system that does not employ spring brake control

valves, the relay emergency valve should function and

apply the brakes.

warning

Serious air leaks in the trailer's braking system

are hazardous conditions that require the trailer

to be placed out of service until they are

properly corrected.

A schedule for periodic cleaning, inspection, adjustment

and lubrication of brake equipment should be estab-

lished by the operator, based on past experience and

severity of operation.

BRAKE SHOES

Brake shoe designs vary, depending on the type of

brake and brake manufacturer. Some brakes require

special tools. If you have problems removing brake

shoes, refer to the brake manufacturer's manual.

BRAKE SHOE SPRINGS

Replace weak or unmated brake shoe springs whenever

they cause pulling or dragging brakes. The springs are

constantly expanding and contracting in the confined

hot area of the brake drums. Excessive heat during

expansion will cause them to weaken.

BRAKE LINERS

The certified Gross Axle Weight Rating (GAWR) in many

cases is determined by the friction level of the brake

liners. The friction level is determined by the axle and

vehicle manufacturer to provide the required brake

torque as prescribed by governmental regulations.

NOTICE:

When replacing liners, be sure to use

liners with the same friction level as those removed

from the trailer so that the GAWR is not reduced.

REFACING DRUM

It is not a good practice to reface brake drums. Refacing

can weaken the drums, making them unable to dissipate

all the heat generated by brake applications, and making

them susceptible to distortion and heat cracks.

If brake drums must be refaced on cam-type brakes,

when oversized linings are used, precautions regarding

cam travel should be checked to prevent sticking cams

or cam rollover. This problem often exists when the

linings become worn. It may be necessary to install

oversized rollers to prevent this problem.

OVERSIZE LINERS

To compensate for material removed when refacing

brake drums, X (1/16" oversize) and XX (1/8" oversize)

liners are furnished by the lining manufacturers. When

oversize liners are used the drums should be machined

in increments of 1/16" (radius increased 1/16").

warning

Enlarging the drum diameter may permit the

cam to rotate beyond its maximum lift when the

lining becomes worn. This can result in sticking

cams or cam rollover.

To avoid this condition and to obtain maximum

lining wear, oversize roller cam followers or

wear plate shims (depending on the brake

design) should be installed when the liners

become approximately 50% worn.

BRAKE SHOE ROLLER CAM FOLLOWERS

Roller cam followers are available in four sizes, each

having been designed for a specific purpose.

STANDARD SIZE (1.250")

Standard rollers are used when installing standard

lining with standard drums, X lining with 1/16" oversize

drums, or XX lining with 1/8" oversize drums.

If the drums have been refaced and oversize rollers later

installed, the oversize rollers must be removed and

standard rollers installed when the new liners are

installed.

1/8" OVERSIZE (1.375")

Use 1/8" oversize cam followers when you install

standard liners with worn or trued drums that are 1/32"

oversize. Circle grind the liners a few thousandths less

than the drum diameter.

brake adjusters

AUTOMATIC ADJUSTERS

Several different brands of automatic adjusters are

common. Because adjustment differs, refer to the

maintenance and adjustment information from the

manufacturer.

Some automatic slacks are equipped with an adjust-

ment pawl which eliminates loss of adjustment. This

pawl must be removed prior to backing off on the

adjustment to prevent damage to the splined pawl

and adjusting screw.

1/4" OVERSIZE (1.500")

Use 1/4" oversize cam followers when X liners have

been installed with 1/16" oversize drums and standard

rollers. The standard rollers should be removed and

1/4" oversize installed before the lining becomes worn

to the point when the brake cam is no longer effective.

1/2" OVERSIZE (1.750")

Use 1/2" oversize cam followers when XX liners and

1/8" oversize drums have been used. The standard

rollers should be removed when the liners become

approximately 50% worn, and 1/2" oversize rollers

should be installed.

CAM PLATE SHIMS

When standard liners are installed with worn or trued

drums, or when X or XX liners have been installed and

are approximately 50% worn, shims or a combination of

shims should be installed under each of the cam wear

plates to permit the cam to be returned to the full

release position. Shims are available in 1/64", 1/32" and

1/16" thicknesses.

BRAKE DUST SHIELDS

The brake spiders used for mounting the brake shoe

assembly parts also become the mounting for optional

dust shields.

The use of dust shields should be determined by

conditions encountered by the trailer. Operations in

mud, sand, dirt, water, and other forms of foreign

materials should be the governing factors. When these

conditions exist during winter months and maximum

cooling is desirable during summer months, dust

shields are easily installed or removed.

NOTICE:

Failure to use dust shields during months

where gravel chips, etc., are used on roads may

allow drums to be scored by these materials. Dust

shields may provide the desired protection.

NOTICE:

Automatic adjusters may seem to be

over-adjusting and may seem to be more sensitive

until brakes are properly burnished.