When driving back from the Mt St. Helens viewpoint the road was winding and down hill, so was using the brakes heavily. When we got to the bottom there was a campground there so we pulled in to check it out. As we were driving slowly around the campground I pressed on the brake and it went to the floor. Pulled into a camping site to investigate. Turns out coming down the mountain the brakes and master cyc were heating up but as long as we were moving down the road they stayed cool enough but once we pulled into the campground they overheated and partially melted the master cylinder piston. We spent the night there and after everything cooled down we got back 40% of the braking back. Limped home with that and the emergency brake when needed. Mechanic told us about the plastic piston and said it happens a lot. 90 F250HD