soft and hard brake pedal????

[bdydrp97s10]

ive replaced all the hard line and rubber lines on my 68 c10, put a new master cylinder on it

and when the truck isnt running it has a good brake pedal, but when i start the truck and the booster has vacuum the pedal gets soft for some reason..

its nowhere near as hard as when the truck isnt running, the pedal goes almost all the way to the floor and then i can feel the pedal pressure a little bit..

im scared to take the truck on its maiden voyage with the brake pedal like this... any ideas what is wrong

and the lines are beld, they have been bled like 4 times just to make sure.. no air bubbles are coming out anymore.