1979 Chevy Camaro Cannot get brakes to bleed ( I.E. No br

I have purchased every thing new includeing
master cyl, proportioning valve, and all brake parts
for front disc and rear drums. This car has been sitting since 1997 and I am now restoreing it.
At first I had good brakes at the disc side but
nothing to the rear. After preping the master cyl.
via bench bleeding to remove all air bubbles, upon
installation of the master cyl when I lightly depress
the pedal air bubbles are again present in the
rear section of the master cyl. That feeds the drum brakes. The dash light is not on so I am sure the
proportioning valve is centered to allow fluid to
the front and back.I removed the line that goes
between the proportioning valve and the junction
mounted to the rear axle and it is clear.I am not sure what to do next.I used both a vacuum pump and
the standard method of fluid in a jar with the bleeder hose below the level of the brake fluid in the jar. Any Ideas? Thank you for your time
Sincerely
John Burkhalter
burkhaltej@sbcglobal. Net