1997 GMC Sierra 5.7 changed master cylinder calipers and brake pads. I have a hard pedal until I turn the truck on and the pedal goes to the floor? Is there something I am missing? I have bled the brakes and gravity bled the brakes only the back brakes will gravity bleed.

possiable brake booster. if you hear a hissing sound and truck runs rough if not possiable master cylinder.

I would go with not bled correctly

The booster should hold up with a hard pedal until
you start the engine, then vacuum causes it to move
down when pressing on the brake

does it stay at floor if tou pump it?

did you try a brake booster ?

Did you bench bleed your master cylinder before you installed it? If not you may not be able to purge the air from it.

George

MAKE SURE YOU HAVE NO AIR IN SYSTEM,YOUR MASTER CYLENDER MAY HAVE WORN SEALS,REPLACE WITH NEW OR REBUILT UNIT.

It's very possible to be the master cylinder, the o-rings on the piston could be worn, letting the fluid seep back through instead of getting full pressure through the lines. Could also be the brake booster.
Try changing the master cylinder first, which is cheaper and easier, and see if there is a difference, you'll need to bleed the brakes again.
To check the booster,
With the engine off, pump the brakes until the pedal is hard, then hold pressure on the brake pedal and listen for air escaping, or the pedal goes slowly to the floor, Do that after the master cylinder has been changed, at least that will be eliminated.

air in the brake lines can cause that, bleed the brakes out, not very difficult if u know anything bout brakes, i can give u steps if u dont know how

pump the pedal about 10 times and see if it comes back up, u need to pump the caliper pistons out after a brake job.

It might be a ALB sensor. Did you check to see if the sensor is operational? Its the cheapest place to check.