Master cylinder or booster possibly gping bad?

[GM-power]

My brakes have been working normal until this morning. Crank the truck this morning and let it warm up( it was sitting outside in the cold) start driving and the brake pedal os super soft. Sometimes ail braking the pedal would lose "pressure" and go all the way to the floor. Bled the brakes this afternoon and there wasn't any air in the lines. Bleeding them didn't do anything. Don't have any leaks anywhere. My rear pads are pretty worn but I didn't think that would make the brakes do that. Anybody got any thoughts? This happened out of the blue so im kind of clueless.
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