[jocko]

A couple notes, some have already been highlighted. While it likely has nothing to do with bleeding problems, there are a few issues with your setup:
- a 69 C10 MC is not a disk brake MC. 71+ were disk MCs.
- on a 1/2 ton truck MC, the front reservoir feeds the front disk (or front drum) brakes. If disk, front lines should be 3/16"; if drum, front lines should be 1/4". The rear reservoir feeds the rear drum brakes and should use 1/4" lines.

If I'm interpreting your first post and picture (flipped below) correctly - you currently have the front res going to the rear drums with 3/16" lines - and the rear res going to the front disks with 1/4" lines - is that correct? While that may (kinda) work once you get the bleeding issue sorted out, it won't work as it was designed. Can you confirm this is actually the case?

If it is, recommend:
1) replace MC with a disk brake MC (recommend doing this regardless)
2) run 1/4" lines to rear drums (from the rear MC res)
3) run 3/16" lines to front disks (from the front MC res)

Sorry, not what you want to hear I know. Again, pls confirm I'm reading your post correctly - quite possible I'm mininterpreting.

Also, is that a residual or proportioning valve? If the latter, both front and rear brake lines from the MC normally go into a prop valve as it is designed to balance front/rear bias. Not sure what I'm seeing there though, so just asking.