Be careful spraying the WD-40 on the master cylinder cap. There is a vent hole on the cap. If the WD gets into the master cylinder it will contaminate the brake fluid and you'll need to flush the system. The original caps have a square top metal cap that you can get a open end wrench on from under the truck. If you have a replacement master ,the cap may be plastic with two wings on the cap that will loosen with pliers. The plastic caps never freeze onto the master. You must have the steel cap. Have patience, you're dealing with an old truck, and they can be stubborn.