Reconfiguring the ABS and Air Brake System on a 1998 International 4900

[MikePhua]

The International 4900 and Its Role in Fleet Evolution
The International 4900 was a workhorse of the late 1990s, widely deployed in municipal fleets, vocational trucking, and utility services. Built by Navistar International, the 4900 series featured a robust chassis, mechanical simplicity, and compatibility with a range of diesel engines including the DT466. By 1998, many units were equipped with Bendix ABS systems, though implementation varied depending on production date and intended use. Some trucks built in early 1998 were delivered without ABS, reflecting a transitional period in federal safety regulations.
Identifying the ABS Modulator Assembly
One of the more confusing components in the 4900’s air brake system is the ABS modulator valve assembly. In some configurations, this includes a Bendix electronic control module mounted atop a relay valve. Severe corrosion and cracking can obscure part numbers, making identification difficult. The lower valve may appear to be a standard relay or quick-release valve, but its integration with ABS logic means it cannot be replaced with a generic part without reconfiguring the system.
Technicians working on retrofit or restoration projects often encounter broken bolts, missing markings, and degraded wiring. In one case, the electronic module was salvageable, but the valve body was cracked beyond repair. Without a clear connection between the electrical and pneumatic sections, confusion arises about whether the component is part of the ABS system or simply a parking brake relay.
ABS Removal and DOT Compliance
For trucks built before mid-1998, ABS was not federally mandated. In some jurisdictions, especially rural or agricultural areas, inspectors may allow ABS removal if the system is non-functional and the truck is otherwise roadworthy. However, DOT regulations require that no ABS warning lights remain illuminated or visibly covered. All associated wiring, sensors, and control modules must be removed or properly terminated.
Recommendations for ABS removal:

• Disconnect and remove all modulator valves, wheel speed sensors, and control modules
  
• Plug or cap unused ports on the tractor protection valve
  
• Ensure no fault codes or warning indicators remain on the dash
  
• Reconfigure air lines using standard relay valves such as the Bendix R12 or QR1C
  
• Confirm brake balance and anti-compounding logic using SR-7 or equivalent valves if needed
  

Converting from Tractor to Straight Truck Configuration
When converting a semi-tractor to a single-axle dump or straight truck, the air brake system must be reconfigured to eliminate tractor protection logic and trailer service lines. This involves removing the tractor protection valve and rerouting air delivery directly to the rear axle.
Key components to retain or replace:

• Dual-circuit foot valve (controls front and rear service brakes)
  
• Relay valve for rear service brakes (typically R12-green)
  
• Quick-release valve for parking brakes (QR1C)
  
• Optional SR-7 modulating valve for anti-compounding
  
• Parking brake control valve (MP-3 or equivalent)
  

In one conversion project, the technician used a diagram from a Ford F800 as a reference, which lacked ABS and used a simplified valve layout. While not identical, the logic was transferable. The technician retained the tractor protection valve for future trailer use but plugged the delivery ports to prevent air loss.
Brake Chamber Sizing and Balance Considerations
Mixing brake chamber sizes across axles can disrupt brake balance and compromise safety. A Type 16 chamber has 16 square inches of diaphragm area, while a Type 24 has 24 square inches. At 30 psi, the force difference is significant—480 lbs vs. 720 lbs. Installing larger chambers on the front axle increases braking force disproportionately, potentially causing premature lockup and loss of steering during emergency stops.
Best practices:

• Match chamber sizes per axle
  
• Use identical slack adjusters and pushrod lengths
  
• Verify brake timing and application pressure
  
• Avoid reusing parts from donor trucks without confirming compatibility
  

In one case, a technician attempted to reuse nearly new brake cans from a parts truck, only to discover they were oversized for the front axle. The imbalance was corrected after realizing the slack adjusters matched but the chambers did not.
Lessons from the Field
During a rebuild, a technician mistakenly installed left and right S-cams on the wrong sides of the rear axle. The error wasn’t discovered until slack adjusters failed to align during reassembly. Correcting the mistake required removing hubs and reorienting the cams—a frustrating but common oversight.
Another operator noted that DOT inspectors in his region were increasingly lenient with pre-2000 trucks, recognizing that parts for older ABS systems were becoming obsolete. However, they remained strict about visible warning lights and incomplete removals.
Conclusion
Reconfiguring the ABS and air brake system on a 1998 International 4900 requires a blend of mechanical insight, regulatory awareness, and practical adaptation. Whether converting from tractor to straight truck or removing a failed ABS system, technicians must ensure that all components function safely and comply with inspection standards. With careful planning and attention to valve logic, the 4900 can be restored to reliable service—without the complexity of outdated electronics. In the world of vocational trucks, simplicity often wins, but only when executed with precision.