well i was backing up then when i apllied the breaks the rear right break suddenly locks can anyone give me some advice as to do to fix this problem its also making noise like scraping at low speeds

you should probably check the bolts holding the caliper to the mount. I would wager one is either missing or very loose. does it shudder at all while you are driving normally? it sounds like the classic case of a drum brake that is not properly installed. Either way, it wouldnt hurt to check the brakes in the rear.

A leaky wheel cylinder can cause the brake linings to swell and grab also. Definitely pull it down and have a look.