Section 71 (Mechanical Brake System - Base (Service Information)): Valve-Proportioning: Operation
WARNING: This page is about a different car, the 2006 Chrysler PT Cruiser. However, it is still accessible from the selected car via links, so may be relevant.
Proportioning valves balance front to rear braking by controlling the brake fluid hydraulic pressure to the rear brakes above a preset level (split point). Under light pedal application, the proportioning valve allows normal fluid flow to the rear brakes. Under higher pedal effort, the valve reduces fluid pressure to the rear brakes. This helps prevent rear wheel skid tendencies.