Truck Brake Systems

Highway Tractor Air Brake System with ABS

Trailer Air Brake System with ABS

Air Brake Sub-systems The air supply circuit Primary circuit Secondary circuit Dash control valves–parking/emergency circuit Trailer circuit Foundation brakes

Pre-ABS Tractor, Trailer, and Dolly

Supply Circuit Components

Typical Air Compressors

Shop Talk FMVSS No. 121 requires that governor cut- out be set within the range of 115 psi to 135 psi. The ATA TMC recommends that governor cut-out be set at 125 psi. It is a legal requirement that governor cut-in occur at no more than 25 psi less than governor cut-out pressure. If the difference between governor cut- out and cut-in is less than 20 psi, the result is too frequent cycling of the compressor loaded and unloaded cycles.

Typical Air Dryer Installation

Shop Talk Air dryers must be fitted with a safety valve. Because the system safety valve is located in the supply tank, if the hose from the dryer to the supply tank got plugged (with ice or contaminants) or kinked, the governor would not cut out the compressor effective cycle, and the resulting high pressure could explode the air dryer.

Warning Alcohol evaporators MUST be located downstream from the air dryer in the supply circuit. Alcohol will turn air dryer desiccant into mush if pumped through the system.

An Older Truck’s Service and Parking Brake Circuits

Tractor Emergency Circuit Pre-ABS System

Pre-ABS Spring Brake Valve Circuit

Dash Control Valves

Shop Talk Make sure you understand the function of each dash control valve in the system. Block the wheels and get into a truck with a fully charged air system. Use the controls and observe the effect.

Tandem Axle Trailer Service and Parking/emergency Circuits

Foundation Brake Types S-CAM WEDGE Disc

Cam Geometry “S” Cam “Flat” Cam

Brake Shoe Rollers

Wedge-actuated Brake System

Air Disc Brake

Dual Circuit Application/Treadle Valves

Typical Dual Circuit Brake Valve

Shop Talk One OEM survey of warranty returns indicates that over half of the foot valves submitted function perfectly. This misdiagnosis results from a failure by the technician to understand how the valve operates normally. Take some time to understand exactly how this important valve masters the operation of the service brakes.

Relay Valves

Primary Circuit

Quick Release Valve

Ratio Valves

Caution If a tractor is converted to a straight truck, the bobtail proportioning valve must be replaced with a non- proportioning relay valve. In a case in which a long-wheelbase tractor was converted to a wrecker, the operator complained of lazy service braking until the treadle was floored, when the brakes would lock. Replacement of the bobtail proportioning valve with a standard relay valve rectified the problem.

Caution Because air is delivered to the trailer hold-off circuit before the trailer tanks are charged, the trailer can be moved before any service braking is possible. You should charge trailer tanks before attempting to move the trailer.

Spring Brake Circuit

Shop Talk Current spring brake control valves must prioritize air to the hold-off circuit over air to the air tanks. This means that when air builds up in the air system, it is first delivered to the hold-off circuit, enabling the release of the parking brakes. This enables a vehicle with a disabled air supply system to be moved much more quickly. A much smaller volume of air has to be transferred to the vehicle before release of the parking brakes becomes possible.

Tractor Protection Circuit

Shop Talk FMVSS No. 121 requires that the Tractor protection valve isolate the tractor air supply from that of the trailer when pressure drops to between 20 and 45 psi.

In-line Single Check Valve

Double Check Valve

Air Brake Chambers

Cutaway of a Typical Air Chamber

Typical Spring Brake Chamber

Caution The spring force in a spring brake chamber can kill. Never take any risks with spring brake chambers.

Spring Brake Chamber Operation

Manual and Automatic Slack Adjusters

Stroke-sensing Automatic Slack Adjuster

Shop Talk Slack adjusters without a grease fitting should not be assumed to be functional. Slack adjusters should be lubricated with a low-pressure grease gun.

Note Around 95 percent of braking on a loaded tractor/trailer combination involves brake application pressures of 25 psi or less; 85 percent of braking involves brake application pressures of 15 psi or less. axle and hub removal Inspection brake install