Bubbles in Master

[N9nefingers]

Welp I've searched and found some threads on this but no one seems to have posted the solution. I re-did my brake system in the 70 C10. Went with disk up front, left the drums in the rear. New booster, master, prop valve. I bled the master on the bench and bleed the brakes but the pedal sucked. I keep bleeding but nothing really improved so I figured I'd start back at the beginning. This time I have the master still on the truck, I bent new lines from the prop valve outlets to the reservoirs and started to bleed. I get no bubbles from the lines themselves but I get several small bubbles from the rear reservoir, the hole that the fluid leaves the reservoir itself. This seems like it should not happen. But no matter how many times I pump the brake I get several small bubbles coming from that hole. Ideas?