I recently had the same problem with my 6.3. After freeing and cleaning the pump it still would not pump. The cause was a coked in-tank screen and steel fuel supply line built into the tank. Cleaned the screen in good carbureator solvent and freed the supply line with a coat hanger from the pump side to the tank sump and let fuel flow through to clean the line of deposits. It now pumps (and the engine purrs) but will need to have the tank cleaned and will change the primary fuel filter to ensure clean fuel.