Heavy Duty trucks use air-actuated brakes rather than hydraulic brakes. And on the big rigs, any loss of air
pressure in the system causes the brakes to apply.
Federal Motor Vehicle
Safety Standard (FMVSS) 121 requires this kind of safety backup to prevent
runaway trucks from careening down the highway.
DOT says 1/4 inch or less pad needs to be replaced. On most pads there
is a hash mark on the side of the pad, giving very easy visual reference as
to when the pads are due to be changed
Making Replacements and Adjustments:
click image to enlarge.
If the old linings are worn to minimum service specifications, or are
contaminated with grease or oil, they must be replaced. This includes all
shoe attaching hardware, anchor pin bushings, oil seals and cam bushings. Anchor
pins, brake rollers and the bushing area of the cams should all be lubricated
with high temperature brake grease.
Inspect and replace if necessary, torch damage
around anchor holes, bent, damaged or cracked ears around anchor holes or
cams, enlarged or worn anchor holes, bent or worn shoe table, elongated
rivet holes in shoe table, or broken welds. Inspect all air hoses that lead to the brake chambers, and replace any hoses
that are cracked or damaged. Check the brake chambers (brake chambers need
to be replaced in pairs if needed, do
not mix parts from different manufacturers, and do not mix long stroke chambers with standard stroke
chambers to make sure they are fully releasing when the brakes are not
applied. Inspect drums, resurfaced or replaced depending on their condition and the amount of wear. Worn or damaged brake drums can be
extremely dangerous. As a rule, drums should not be returned to service if diameter wear exceeds 0.080 in., if the drum has hard spots (blue,
discolored areas) or cracks across the drum braking surface. Inspect and
pack wheel bearings. Inspect air dryer it takes the air from the compressor and removes most of the moisture and contaminants before the air enters the “wet tank” (a storage
tank that acts as a pressure reservoir and also a secondary trap for moisture). The dryer on turbocharged engines also has an “isolation valve”
to prevent loss of boost pressure if the compressor receives air under pressure from the turbo. Mounted on the wet tank are a “pop-off valve” (a safety valve that vents air if tank pressure exceeds 150 psi), a “low pressure switch” to warn the
driver of low air pressure, and an automatic or manual “drain valve” to drain moisture from the wet tank.
Check S-cams for wear at the bushings, head and spline areas. Check slack
adjusters for proper settings and operation. Check clevis pins and slack
bushings for wear. If wear is over 0.030 in., you need to replace both. Never mix manual
and automatic slack adjusters, and never use slack adjusters from different
manufacturers on the same axle.
Airing It Out: click image to enlarge.
Air pressure is generated by an engine-driven compressor, which is also supplied with oil,
coolant and filtered air by the engine. The compressor’s output on newer trucks (1992 models & up) is regulated by a governor. Federal Motor Vehicle
Safety Standard requires a
cut-in pressure of at least 85 psi, but 105 psi is recommended to handle trailer brakes.
SECONDARY SERVICE SYSTEM:
This part of the air brake system controls the brakes on the front
steering axle of the tractor. It includes a “dry” tank for air storage (with a one-way check valve and pressure gauge), a “dual
foot control valve” that allows the driver to modulate the application of the vehicle’s brakes, a “quick release valve” to
release air from the service chambers in the front brakes, “service chambers” which use air pressure against a diaphragm to apply the
brakes via a push rod, and slack adjusters (manual or automatic) for adjusting the brakes.
INSPECT PRIMARY BRAKE SYSTEM:
This part of the air brake system also has a separate “dry” tank (with
gauge and one-way check valve) and operates through the dual foot control valve. Air is routed to the drive axle brakes via a “service
relay” which modulates both application and release. The spring brakes provide both service and parking braking. Manual or automatic
slack adjusters provide brake adjustment.
INSPECT PARKING BRAKE SYSTEM:
A yellow diamond knob “dash valve” allows the driver to release and apply the parking brakes. A “quick release valve” on 1992 and newer
trucks allows the parking brakes to be fully applied in three seconds or less. The spring brakes are applied when there is no air
pressure in either the parking or service chambers of the unit. When air is applied to the parking side only, the brakes are released.
When air is applied to both sides, the brakes are reapplied to stop the vehicle. A “two-way check valve” monitors the pressure in the primary and secondary system and delivers
whichever pressure is higher. This devise is required by FMVSS 121 so that anytime air pressure is lost in either the primary or
secondary service system, the vehicle’s parking brakes can be released at least once.
INSPECT ANTI-COMPOUNDING SYSTEM:
A “quick release two-way check valve” is used to prevent the parking brakes and service brakes from being applied simultaneously. Not all
vehicles have this.
INSPECT EMERGENCY SYSTEM:
An “inversion valve” is required on all straight trucks es, and is optional on tractor/trailers. This system comes into play if air
pressure is lost in the primary service system. It gradually releases air from the spring brake parking chambers so the vehicle
can come to a safe, controlled stop.
INSPECT TRAILER SUPPLY SYSTEM:
A second knob on the dash, color coded red and shaped like an octagon,
controls the application and release of the trailer’s parking brakes, plus the filling of the trailer service reservoirs with air
pressure. The trailer control valve may be part of a “module dash control valve” that includes the yellow parking brake knob for the
tractor. A protection valve isolates the trailer air lines from those on the tractor to prevent loss of air
pressure in the tractor in case of a leak in the trailer’s system.