hi i own a 2006 chevy silverado with 5.3liter 2wd, i have started to rebuild the brake system, new calipers, brake lines, master cylinder, wheel cylinders.
when the truck is not running i get great brake pressure. As soon as i turn on the truck pedal goes to floor. i have used a scan tool and auto bleed, then bleed all 4 wheels using the 2 person pump method serval times. any help would be great

 

Hopefully this will get moved to the right section. I think there are others who can give you an exact answer, but first:

Did you bench bleed the new master cylinder first? If not you have air in there, and you will need to remove that and bench bleed it, reinstall, and bleed the brakes again.

If you've already done that, my guess is the power steering pump. Mine goes to the floor, then when I start it, the pump kicks in and the pedal is firm as can be. (I added hydroboost though, and not sure about your setup).

 

so i took off the master and bench bleed, then auto bleed abs pump,then bleed each wheel started with passengar rear. still same issue. i have not replaced the brake booster or the abs pump.