There is a rubber cap on one end, or both ends. Remove the cap and you will see a pin with a flat nail head. Lift that nail head to bleed the brakes. There are some valves that don't have a nail head, those need too be pushed in. They make a small clip tool to lift the pin. They also make a clamp tool to push the pin. If the proportioning valve has a pin each end, they both need to be moved.