I bleed the brakes many times completly they were perfect but after I started up the truck after ten or fifteen minutes air got back into my brakes do you think its a bad master cylinder?

 

Could be a pressure leak from the power brake booster that's blowing past the seal in the master cylinder. I'd replace both if I could find nothing else ( like a brake line connector messed up or a damp-looking brake line or an old (hairline cracks when flexed) hose).