Brake and carb help

[Dz1331]

First time to post. So thks in advanced for any help.

Ok have a problem. I have power brakes. Replaced my master cylinder bleed it and did a good bleed on all 4 corners. I have real good pedal when the truck is off, but when I start it and put it in gear then push the brake pedal it goes to the floor. Master cylinder never loses fluid. What next?

Also next problem bought the truck has a crate motor runs real smooth but when I kill it and come back a few days later it will not crank. Put a little gas in the carb and cracks right up and runs smooth again. Original carb. From what I read y'all know everything about these trucks. Need a little help.