1974 C20 Brake problem

[mntoy]

I have a problem bleeding the brakes, the master and vacuum power booster are both new and came bolted together as a single unit. (rear drums front disc brakes) I bleed the rear brakes and the pedal feels firm and all the air goes away when bleeding. Then I move to the front brakes and I can hear a swooshing sound from near the Poportion valve (sounds like lots of air) and I cannot get all the air out of the front brakes and the pedal gets soft and hard as I continue to bleed the brakes, but never as hard as when I did the the back brakes first. After a quart of bleeding I finally gave up and started the truck and pushed the brake pedal down, it went right to the floor and did not return and a groan came from the vacuum booster. After I shut the truck off a minute goes by and the pedal returns to normal position. When the truck is off the pedal seems firm, but when I return to bleeding I cannot get the air out of the front brakes. Any ideas how to fix this problem, any help would be great. Mike