Why does my brake pedal goes from soft to hard when im driving for about 10 to 15 minutes what could it be?

Have mechanic test drive .. Possible 'master cylinder' or brake booster problem.

Do the brakes start to lock up?

Next time you enter the vehicle , do not start... then pump brake pedal several (at least 4) times and then HOLD the pedal firmly. Now with pedal held , start the car. Does the pedal sink? Now take the results of the steps I had you do , and go to a qualified repair shop for an estimate. Most likely you just knocked out 80% of their diagnostic , saving you money & them time wasted looking for the wrong problem. Be sure also , to inform them of the previous work and who did it(shop or personal). Good luck. How does it run? Vacuum leak/check valve could also be a possibility.