1984 F250. New master cylinder. I replaced the master cylinder because fluid from the small reservoir was transferring to the larger reservoir. I replaced rear wheel cylinders due to age and crud, new rear shoes, all new hardware. I vacuum bled the system and have a solid high pedal with no vacuum assist. With assist the pedal will drop to less than half way to the floor (good solid high pedal). All seemed normal to me until I check the fluid level after driving about 300 miles. Same problem in the master cylinder. Fluid leaves the small reservoir and migrates to the larger reservoir. The only thing I can think of is bleed over, either in the MC or the Combination valve. Has anyone else experienced this? I could not find any reference to this particular problem in older posts.