Yeah, if it don't come loose that way, you may have to pull the brake pedal off and take some fine sand paper or emery cloth and get the rust off. Then, just put a little white lithium grease on the shaft and put it back on.
Does it have drum brakes or disc brakes? If it has drum brakes, it won't have a master cylinder, just a cable. The cable could be stuck. It might need replaced or the brake shoes might be seized up.Maybe the master cylinder is seized up.
Yeah, I agree with mud. Your pedal has seized up. Try what mud said and if that don't work, you may have to remove the pedal, clean the rust off with sandpaper or emery cloth, and then put some white lithium grease on the shaft.I would spray some PB Blaster on it and try working the pedal up and down with your hand and see if it will work loose. Sounds like its froze up or in a bind.