Description & Operation

Brake system is a dual hydraulic pedal-actuated system with a master cylinder, proportioning valve and pressure differential warning switch, vacuum brake booster, front disc brakes and rear duo-servo drum brakes or optional rear disc brakes. Hydraulic brake line routing is split front and rear for safety reasons.

Master cylinder has dual pressure chambers with a primary piston to supply hydraulic pressure to front brakes and a secondary piston to pressurize rear brakes. There is one outlet for both front brakes and one outlet for both rear brakes. A quick take-up valve is included on master cylinders, to provide a large volume of brake fluid during initial brake application.

Hydraulic system pressure differential is controlled and monitored by combination valve, located below master cylinder. Combination valve performs the following functions:

• Limits amount of pressure applied to rear brakes to allow most pressure to be applied to front brakes.
• Transfers hydraulic pressure to front brakes if pressure loss occurs in rear brakes and vise versa.
• Closes contacts in pressure differential switch to turn on BRAKE warning indicator light.
• Metering valve portion of combination valve prevents hydraulic pressure from being applied to front brakes until rear brakeline pressure has overcome rear brake shoe return spring tension, and brake shoe-to-drum clearance has been taken up.

Front disc brakes are continually self-adjusting. Caliper piston seals are designed to retract pistons enough to allow brake lining to lightly brush rotor. Sliding caliper design compensates for lining wear. Replace brake pads in axle sets when lining thickness is .030" (.76 mm) or less.

Rear disc brakes are a single piston caliper assembly. Caliper piston seals are designed to retract pistons enough to allow brake lining to lightly brush rotor. Sliding caliper design compensates for lining wear. Parking brake system uses brake shoes located inside rear brake rotors. Replace brake pads in axle sets when lining thickness is .030" (.76 mm) or less.

Rear drum brake assembly is a duo-servo design. When brakes are applied, wheel cylinder force applied to the primary shoe is multiplied by primary shoe friction against the drum. This multiplied force is applied to the secondary shoe, which stops the drum and rear axle rotation. Adjustments for both rear brake primary and secondary shoes are automatic during reverse brake applications. Replace brake shoes in axle sets when lining thickness is .030" (.76 mm) or less.