I have a 1992 ford f-350 7.3 diesel single rear wheels and 2 wheel drive. I have changed the brake booster, the booster check valve, the master cylinder, the two rear wheel cylinders, both calipers, and both rear brake lines. I bled the brake system twice now the second time with a bleeder kit. Now I am having an issue where when you step on the brakes sometimes it feels like you are stepping on a brick and you have to push really hard to get the brakes to do anything. But other times it acts just fine. I found out from ford there is no proportioning valve on this truck does anyone know what might be happening later this month I need to pull a fifth wheel trailer and don't want my brakes having issues.