I keep the tube in an arch-shaped loop so air can't go back down into the bleeder valve. Any air should rise up and away from the bleeder valve. As the fluid goes back down, from the top of the arch into the bottle, it should create a slight vacuum at the bleeder valve. At least that's my theory.....
On the threads of the bleeder valve, I think what happens is the first stroke of the pedal forces fluid around the threads and it becomes a wet seal. Releasing the brake pedal slowly prevents the sucking of air back through the threads. At least that's my theory.....
I also ran a little check test. I have the rear end of the car up in the air with the wheels off. So I pressed the brake pedal down and held it in place with a bar. Both rear wheels will spin freely. The shoes barely touch the drum.
I can lock the brakes if I turn the manual adjuster far enough, so I know they can work. I'm not sure if that confirms air in the system or if the brake cylinders are both bad.