jammed

anyone expereinced it before? i havent used my rear brake for ages as it seems loose(no grip at all)..however, a week ago, it suddently jammed and i nearly got rammed from behind.. i stop, wait for a while and bleed it..it seems ok but after about a kilometer, it got jammed again. So, i get...

havent use my rear brake for ages and it suddenly jammed this morning and a car behind me nearly rammed me? what caused it?