Mack Truck Brake Service Manual

Air Brake System MVSS Requirements MVSS Compliance
As manufactured by MACK Trucks, Inc., the air brake system on MACK chassis (both incomplete and complete) comply with the applicable requirements of U.S. Federal and Canada Motor Vehicle Safety Standards (MVSS) 106, Brake Hoses, and 121, Air Brake Systems. Any change or addition to the system may cause the vehicle to no longer be in compliance with these MVSS. MVSS 121 requirements cover (but are not limited to) the following:

  1. Air compressor build-up time
  2. Air reservoir volume
  3. Service brake stopping distance
  4. Brake actuation time
  5. Brake release time
  6. Parking brake hold on grades
  7. Emergency brake stopping distance

Air Brake System Truck Tractor
There are basic differences between straight truck and truck tractor air systems. On a straight truck, a spring brake control valve is added to the emergency brake air circuit. This gives the driver modulated control of the spring brakes through the treadle valve in the event of a primary system air loss. Additionally, spring brake chambers are installed on both axles of a tandem rear axle unit so that if there is a partial air system pressure loss, the emergency brake system will stop the vehicle within the required stopping distance, and also to meet parking brake system requirements.

A truck air system is designed to be operated as that of a truck, and a truck tractor air system is designed to be operated as that of a truck tractor. When converting chassis for use other than as originally intended (e.g., converting a truck tractor to a truck), the air system must also be changed to ensure that the vehicle remains in compliance with MVSS. Contact MACK Trucks, Inc. Product Support for more information.

Air-Operated Equipment
Additional air system capacity may be required for air-powered accessories to operate properly without jeopardizing the integrity of the air brake system. Motor Vehicle Safety Standards (MVSS) 121 requires an air capacity 12 times the total volume of all air brake chambers on the vehicle. For additional information on calculating total air volume and brake chamber rated air volumes, refer to Liftable Axle Air System Requirements section in this bulletin. If additional air capacity is required, an expansion reservoir should be installed. The reservoir and piping must comply with MVSS.

Air Compressor Capacity
If increased air system volume is necessary, it is also necessary to determine if the air compressor has the capacity to supply the air system without having to run in the loaded mode (compressing) for long periods of time. Motor Vehicle Safety Standards (MVSS) 121 requires that the air compressor must be able to increase pressure in the supply and service (primary and secondary) reservoirs from 586 – 690 kPa (85 –100 psi), with the engine running at maximum governed RPM, in a specific amount of time, depending on required and actual reservoir capacity. If the existing compressor cannot accomplish this, a larger compressor must be used. First, however, make sure that an air compressor malfunction or other type of problem with the air system is not causing the slow build-up time.

Air Solenoid Valves
MACK chassis now incorporate electrically operated air solenoid valves to direct air pressure to the various accessory air circuits such as air suspension control, fifth wheel slide, inter-axle lockout, power takeoff (PTO), etc. Additional air solenoid valves can be added to the air solenoid valve pack which is located on the right-hand frame rail, mounted behind the intermediate crossmember.

To add an air solenoid to the solenoid valve pack, disconnect the valve pack connector from the chassis harness, and then remove the valve pack from the mounting bracket. Remove the end cap from the valve back by twisting the cap counterclockwise. Engage the new solenoid into the locking ring of the last solenoid in the pack, and then twist the solenoid clockwise to lock the solenoids together. Reinstall the end cap in the same manner.

Air Solenoid Valve Pack Connector and Pin Part Numbers
Should replacement of the valve pack connector body, pins, seals and plugs be necessary, refer to the following illustration for the applicable part numbers.

Liftable Axle Air System Requirements
Installation of a liftable axle(s) may require additional air capacity for operation of the service brakes, up/down air bags and suspension air bags. Note: The addition of a liftable axle increases the load carrying capacity of the vehicle, which may affect the ability of the parking brake system to hold the vehicle on a hill. Motor Vehicle Safety Standard (MVSS) 121 requires that the parking brake system be capable of holding the vehicle under specified conditions. To ensure continued compliance with MVSS 121 when a liftable axle is added, it may be necessary for the installer to increase the capacity of the parking brake system to account for the increase in the gross vehicle weight rating (GVWR).

Mack Truck Brake Repair Manual

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