Volvo Coupes/Sedans/Wagons 1970-1989 Repair Guide

Proportioning Valves


Each of the brake circuits has a proportioning (relief) valve located inline between the rear wheels. The purpose of this valve is to ensure that brake pressure on all four wheels compensates for the change in weight distribution under varied braking conditions.

The harder the brakes are applied, the more weight there is on the front wheels. The valve regulates the hydraulic pressure to the rear wheels so that under hard braking conditions they receive a smaller percentage of the total braking effort. This prevents premature rear wheel lockup and possible skidding or loss of control.


1800, 100 and 200 Series

See Figure 1

Sophisticated pressure testing equipment is required to troubleshoot the dual hydraulic system, in order to determine if the proportioning valve(s) are in need of replacement. However, if the car is demonstrating signs of rear wheel lock-up under moderate-to-heavy braking, and other variables such as tire pressure, tread depth, etc., have been ruled out, the valve(s) may be at fault. The valves are not rebuildable and must be replaced as a unit.

To replace the valve:
  1. At the proportioning valve, disconnect and plug the brake lines coming from the front of the car.
  3. Loosen the connection for the flexible brake hose to the rear wheel a maximum of 1 / 4 turn. This will allow some flex in the hose during removal.

Click image to see an enlarged view

Fig. Fig. 1: Do not replace the brake proportioning valve, unless all other possibilities have been ruled out

  1. Remove the bolt(s) which retain the valve to the underbody, and unscrew the valve from the rear brake hose.
  3. To install the valve, place a new seal on it, and screw the valve onto the rear brake hose and hand tighten. Secure the valve to the underbody with the retaining bolts(s).
  5. Connect the brake lines from the front of the car and tighten both connections, making sure that there is no tension on the flexible rear hose.
  7. Snug the connection to the rear brake hose, but do not overtighten.
  9. Bleed the brake system.