fastfreightliner

Hi guys I'm new to the forum. I have a '94 f150 and my brakes started acting up. I barely touch the brake peddle & it just gets sucked down to the floor and stops on a dime..............damn near puts ya through the windsheild, then you have to pull the peddle back up with your foot cause it wont come back up on it's own. Is it the brake booster or the master cylinder. I dont have any fluid leaks & the brake booster holds a vacuum, not sure if that makes a difference or not. Any help would be greatly appreciated.