my truck while stopped on a hill if take foot off the brakes it rollin back
 

The other day I was driving up a hill stopped at a stop sign when I took my foot off the brakes my truck rolled back about four feet when I noticed I applied the brakes ,then I took my foot off the brakes to see if it will continue to roll back and it did . I checked the fluid level and noticed it was low I put some tranny fluid and still same problem. The tranny shifts all gears, but when on a steep hill it just can't stay in place .Is my tranny bad ? HELP.