Pulling my hair out...HELP...brakes

[GMC-YA68]

Here's my story

Recently acquired a 1973 GMC 3500 dually. Sat for years before I ended up with it. Passenger rear wheel would NOT roll when I got it. We had to winch on and off the trailer.

I first went straight to the fronts and put new calipers, pads, and replaced both rubber brake lines because they were rusted shut and barely had fluid passing thru them.

The truck would "drive" but brakes have always sucked. At this point I haven't done anything with the rears. (Besides hitting the locked up one with a hammer to break it loose)

I then put a new rubber line on the rear, the one that T's to each rear wheel.

Drove around the block and brakes still were very hard. The next night the one of the rear wheel cylinders let go. Ended up replacing both sides. Kept shoes and drums as they were.

Bled everything, still no change. Then the next day...brakes went to the floor, no pressure. I replaced the master cylinder (bench bled it) and installed, brake pedal turned hard as a rock again.

Put a new booster on today. No change at all. Except for a massive vacuum leak sound.(hissing) It's the check valve going into the booster. It is very noisy and causing the truck to raise it's idle 1000rpms. I can pull that and stick my finger over it and the hissing completely disappears and my idle returns to normal.

Pros; what's the fix here.????