Brake light stays on

The brake light is on even though you are not braking. Drivers behind cannot tell when you really slow down, and the light drains the battery: the K-Light manual lists a brake light stuck on as a cause of an undercharged battery (PDF p. 36). Usually the front lever or rear pedal is not returning fully, a switch is stuck closed, or the rear switch is adjusted too tight. Sometimes the brake itself is dragging.

Is it safe to ride?

With care, for a short trip to fix it. First make sure the brake is not dragging: spin each wheel by hand on the stand. If a wheel is hard to turn or the brake is hot after a short ride, do not ride: see Brake drags and the wheel does not spin freely.

Quick checks first

• Push the front lever and rear pedal fully back to rest by hand. Does the light go off?
• Unplug the front switch, then the rear switch, one at a time. Which one turns the light off?
• Is the light only faint (not full brightness)? Then it may be a tail light that looks bright, a twin bulb fitted the wrong way, or an LED unit.
• Was the rear brake pedal or chain adjusted recently? Moving the wheel or pedal can pull the switch spring.

Diagnose it step by step

1Does the light go off when you unplug the front switch?

• Yes: the front switch is stuck closed, or the lever is not returning. Go to step 2.
• No: plug it back and go to step 3.

2. Does the front lever return fully? Check for a bent lever, a dry pivot, a sticky master cylinder or a lever adjuster set too far out.

• Lever sticky or bent: Service brake and clutch lever pivots, and see Brake or clutch lever is loose, bent or broken.
• Lever returns well: the switch is faulty. Set the multimeter to continuity (beep). Switch unplugged, lever released: it must not beep. If it beeps, replace the switch: Test and adjust the brake light switches.

3Does the light go off when you unplug the rear switch?

• Yes: go to step 4.
• No: go to step 5.

4. Is the rear pedal resting in its normal position? Check the pedal return spring and the pedal height. On a disc brake, the pedal push rod must not hold the master cylinder piston in (on the K-Light 125, the manual sets a 0 to 0.2 mm gap between push rod and piston, PDF p. 116).

• Pedal sits low or spring is weak: Adjust the rear brake pedal.
• Pedal is correct: the switch is adjusted too tight or stuck. Adjust or replace it: Test and adjust the brake light switches.

5. Light stays on with both switches unplugged? Then power reaches the brake filament another way.

• Check the bulb. A twin bulb (K-Light 125: P21/5W, PDF p. 99) has offset pins so it fits one way only; if forced in wrongly, the brake filament lights with the tail light. Refit it correctly: Replace a bulb.
• Check for melted insulation joining the tail light wire and the brake light wire, often near the lamp or under the seat: Test a lighting circuit.
• Check any accessory, alarm or LED kit wired into the brake light circuit.

6. Is the brake dragging? If the light was on because the brake was applied, check the brake: Check for brake drag.

Causes, most likely first

Reference values (K-Light 125)