Help! My brakes are applying themselves!

Ok, 1994 Ford F150 I just bought it. Sometimes when I apply the brakes the pedal actually pulls itself down farther then I do.It doesnt lock the brakes and it has never stopped the truck all the way. It will release before that happens. There is a new master cyl. brake booster,pads,rear shoes ect…what do I check next? Brakes also seem very touchy… thanks

The adjustment between the new power brake booster and new master cylinder is incorrect. Get a factory shop manual to see how to set it.