how do you tell if your brake master is bad?

[vardenafil]

How do you tell if your brake master cylinder is bad? When the truck is off the pedal is firm and hard. As soon as I turn the truck on the pedal gets super mushy. I can lock up the wheels but only after the pedal reaches the floor. I have to pull the pedal back up with my foot to get it to return to the normal position. I pulled the master off and did not notice any external leaking into the booster or anything like that.

here is what ive done so far to the brake system:
-I replaced all the soft brake lines on my 72 k20 with braided brake lines.
-I replaced the calipers and pads on all four corners.
- Ive bled and re bled the truck multiple times and no air comes out.
- Ive adjusted the rear parking brake calipers they have almost no slack.

The only thing I havent replaced is the master and the brake booster and the hard steel brake lines.

Does this sound like a master problem? or is there something im missing?