I found a simple way to add a brake light switch to a bicycle brake lever. Specifically, this is for a mountain bike lever that pulls a cable for the brakes although it's possible it could work on other levers. I would not try this on super lightweight brake levers as the metal might not be thick enough to tap threads into and it might weaken the lever excessively. The switches I used can be found at radio shack and are mini momentary pushbutton switches that are normally closed (N.C.). The tools you will need are: allen and/or box wrenches/sockets, duct tape, half round metal file, pencil, tape measure, center punch, drill and bits, 7mm fine tap and tap holder, wire, solder, soldering pencil, heat shrink tubing, heat gun and electrical tape.
Step 1: Prepping the Brake Lever
The first thing you will need to do is remove the reach adjustment screw from the body as shown in the first three pictures. This can be done using a small allen wrench. Next you need to remove the lever (silver part) from the body (black part) if possible. This is usually accomplished by removing a pivot screw and nut or a pivot pin and clip. Pay careful attention to how it goes back together and set the lever and small parts aside. Next you want to wrap the body as shown in the picture with a layer of duct tape to protect the finish when you clamp it in the vise.
Step 2: Prepping the Surface for the Pilot Drill
Next you need to find the best spot to put the switch. It should be 1/4 to 1/3 of the way from the pivot bolt/pin hole to where the lever pulls the cable but there is some room for flexibility here. Make sure that where ever you place the hole, the switch will contact the brake lever when it is released. On some switches , there is enough room next to the reach adjustment screw to put the switch or there is no reach adjustment screw and you will not need to do the filing in this step. In other cases (first picture), you may want to use this screw's hole for the switch hole and you will not need to do the filing in this step . In still other cases (second picture), the reach adjustment screw will be in the way and it will need to be permanently removed and any protruding material around the threaded hole will need to be removed. First clamp the body in the vise and if needed, use a half round metal file to remove any protruding material around the brake reach adjustment screw hole until the surface is smooth and relatively flat. Then use some sandpaper to make the surface smooth. Next, mark where the switch hole will be drilled using a pencil. It should centered between the sides of the body and at least 1/4" away from the center of the brake reach adjustment hole. Any closer will risk the the two holes merging into each other. In the photo, I marked it 1/4" away from the brake reach hole. Once you have the spot use an auto punch or center punch to make a depression so the drill bit will not wander when you drill the hole.
Step 3: Drill Pilot Hole and Tap the Threads for the Switch
The switch I used was a miniature momentary switch with normally closed contacts. These can be bought in packs of 4 from radio shack for $3-4 a pack. The threads on these switches are M7x.75 metric threads so if you use the Radio Shack switches you will need an M7x.75 tap. The final drill bit you need to use before using the 7mm tap will be 1/4". Start with a small bit like 1/8" and drill the hole perpendicular to the surface. Enlarge the hole with successively larger bits till you reach 1/4". Now carefully start tapping the threads using care that the tap is perpendicular to the surface. Use WD40 or kerosene to lubricate the tap and back it out when you encounter resistance to clear the threads. This is soft metal so it should go relatively quickly. Once you are through, remove the tap, remove the body from the vice, remove the duct tape and clean off the body.
Step 4: Screw the Switch Into the Body and If Necessary Trim Material Off the End
First remove the black or red plastic end cap from the switch. It should pop off easily. Now screw the switch into the body to be sure the threads you just made are good. Then remove the switch and assemble the lever onto the body. Now screw the switch in all the way while noting whether it interferes with the lever retracting. If it does, remove the switch and using a file or a grinding wheel, remove a small amount of material from the end of the switch. Screw it back in and if it still interferes with the lever, repeat untill the lever can fully retract. Be careful not to remove too much or the switch will not depress fully to turn off the brake light.
Step 5: Touch Up
Remove the switch and use an appropriate color paint to touch up where you removed the paint. I also filled the hole for the reach adjustment screw with epoxy before painting.
Step 6: Add Wires to the Switch
I used 18 gauge automobile hookup wire from the local auto store for this step. My bike had two brake levers so I cut the two wires long enough to meet up with the ones from the other lever at the bicycle stem. Trim 1/4" of insulation off each end using a wire stripper. Feed the wire through the hole in one of the switch leads going from the inside to the outside. Once through, bend the stripped section in half and then solder it to the lead. Repeat with the other lead. Use heat shrink tubing that just fits over the soldered connection and cut two 1/2" lengths of it. Slip one over each wire till they cover the soldered area and use a heat gun to shrink the tubing in place. Use a larger piece of heat shrink tubing that just fits over both wires at the switch and cut it to 3/4" length. Slide it over both wires till it butts up against the switch and use the heat gun to shrink it in place. Now wrap the two wires in electrical tape to keep them together and protected. Screw the switch back into the brake lever.That's it. Now it's up to you to figure out how you want to wire up a brake light. This tutorial only covers making the brake switch.