While riding the other day I noticed that my rear brake was no longer making my brake light come on so started checking for loose connections. The front brake switch was still functioning as designed. Connections were all clean and tight so I checked out the brake switch with an ohm meter. All good there so following the wiring diagram I ended up at the light bulb monitoring box under the tank with all of the relays. Fortunately I had another K100RS that I could pull the part from and that was it. So..... if one of your brake switches on your K ceases to function while the other continues to work, follow a similar course of diagnosis but consider that bulb monitor if everything else checks out.