Replacing the Brake Lever
A broken or damaged brake lever is easy to replace. Just strip the stuff off the handlebar and put the new one on. The only tricky part is handling the cable -- if you don't have a slot in the brake lever housing to allow the cable to be pulled out of (instead of through) the lever mechanism, you'll need to strip the entire cable from brake lever down to brake caliper. (See our section on cables.)
Use a butter knife to raise up the inside edge of the grip. Pour some water down into the grip.
Begin rotating the grip until it breaks free and slides off. If you need to cut the grip off, no problem. Use a wallboard knife to slice it. You're probably due for a new set of handgrips anyway.
Helpful hint: If you ride a long time in the rain, you may notice your grips become sloppy -- rotating freely around the handlebar. If you want those grips to stay put, slide them using lacquer as your "lube." Once they dry, they're going nowhere. (Of course, you'll have to CUT them off if you need to take off shifters or brakes in the future.)
Next take off your shifter. (Some shifters, for example Shimano Deore XTs, are in a single unit with the brake. If so, take the whole thing off.)Find the hex bolt that attaches the shifter to the handlebar. Loosen it until the shifter slides on the handlebar.
This illustration shows a grip-shift, but an index shifter is removed in the same way. Also remove anything else that's in the way -- a dropper-seat controller or a shock lockout, for example.
Slide the brake lever off of the handlebar.
Grab the cable and rotate it so the slot in the retaining clip lines up with the cable. Now lift the cable out of the retaining clip.
If you don't have a slot in the housing and adjusting knobs, you'll need to remove the cable at the brake and pull it through. This means you'll (essentially) need to do a cable replacement operation.
(If you had to remove the entire cable and cable housing, re-route it down to the brake.)
Now put the new brake lever onto the handlebar. Rotate it so the lever matches the one on the other side, and tighten it with the hex bolt. Tighten up the cable at the retaining clip on the brake caliper, and reattach the cable housing to the brake mechanism.
Reattach the shifting mechanism and handgrip to the handlebars. Test the brake and adjust as needed on the workstand, then take a test ride and adjust the brake again.