'83 300D Turbo

I replaced the rear rubber brake lines and calipers. The calipers were used from the junkyard and the rubber hoses are new. I flushed and bled all 4 corners. Two months later or however long it has been the rear wheels are still getting dirty and when reversing the rear brakes squeak or sometimes when braking. The fronts are cleaner than the rears. I made sure the parking brake is not sticking, which it isn't.

Could the master cylinder be suspect? I'd like toot have to replace the calipers even though I have the originals on the shelf. I could rebuild those(replace cylinder seals) but they were not leaking.

Ideas?