Does your fuse box have power at either side of the fuse for the brake light switch? Are any other fuses having issues. Take a 12V test light (or multimeter) with the clamp attached to ground and test every fuse for power, fuse should be 12V present on one side always except lighting circuits which have to have the lights turned on from the dashboard switch. The Brake light switch will have 12V present at all times on one side of the fuse. If the fuse has 12V on one side and the fuse is good then you probably have a bad break light switch or broken wire which you would have to trace to the tail light lamp sockets, hope that helps.