Power Brake Booster

DESCRIPTION






All 3500 model vehicles and 2500 diesel model vehicles are equipped with a hydraulic booster. The hydraulic booster supplies power assist to the brake system. The booster is mounted to the front cowl panel on a bracket. The master cylinder is mounted to the front of the booster.
The hydraulic pressure is supplied to the booster from the power steering pump. The pressure line from the pump is connected to the booster. From the booster a second pressure line is connected to the steering gear. Return lines from the booster and steering gear are connected to the power steering pump reservoir.
A nitrogen charged pneumatic accumulator on the booster provides reserve power assist pressure. If power steering pump pressure is not available (broken belt/pump failure) the accumulator reserve pressure is used. This provides 2 or 3 stops at partial boost.
The system is designed to permit manual brake application in the event hydraulic pressure is interrupted. A somewhat greater pedal effort is required to apply the brakes manually.

OPERATION
With the brake pedal released most of the hydraulic fluid is routed through the booster power section to the steering gear. A portion of the fluid is diverted into the booster power section, then returns to the power steering pump reservoir.
With the brake pedal depressed, the input rod and piston move forward. This causes the lever assembly to move the sleeve forward to close off the holes leading to the open center of the spool valve. Additional lever movement moves the spool valve forward in the spool valve bore. The spool valve then diverts some hydraulic fluid into the cavity behind the booster piston building up hydraulic pressure. This hydraulic pressure moves the piston and output rod forward. The output rod moves the primary and secondary master cylinder pistons which applies hydraulic pressure to the brake system. When the brake pedal is released, the spool and sleeve assembly returns to its normal position. Excess fluid behind the piston returns to the power steering pump reservoir through the return hose.