After a lot of searching, this is the closest I could find that looks like your MC.
https://www.performanceonline.com/19...R-DRUM-BRAKES/
From the searching, it is likely that you don't have residual check valves in your MC. When you rebuilt the MC from your bag of parts, did you have two of these, one for each port?
If you don't remember, push a paper clip it each port. If you feel resistance right away, you have a check valve in that port. check both front and rear.
Glenn