Help troubleshooting brake issue

[argonaut]

I'm trying to get my 72 Blazer road worthy for the trip home on Friday. The brakes don't seem to feel like other trucks with power brakes.

WHen I apply the brakes the pedal travels half way down and seems to apply maybe half or so of the overall brake pressure. Then even if I STAND on the brake pedal it doesn't seem to move much or apply much more brake force.

I tried removing the vacuum line from the booster, plugging it and driving/applying the brakes. The function was similar but seemed to have to push even harder on the pedal.

I then hooked a different hose to the booster and applied a vacuum with my mouth and it seemed to hold the vacuum. SO I reconnected the vacuum line.

I've also bled all the brakes and flushed with new fluid.

Could this be a bad booster, or an issue with the prop valve? Help!