I have come this types of problems before. The brakes need to be looked after. It is good do refer a mechanic for a complete servicing. Have you bought the car direct from show room or a resale? Due to friction or decrease in brake fluid, brakes generally wear out. Sometimes it needs to be replaced. Regarding seat belt, how many times it happened while driving? If it is once, it might be an not properly locked error. If it is more, try getting a car mechanic.