Step 1: Get Rid Of The Nasty
After a few years bike hand grips get pretty stuck on. The first step to repairing your shifters is to remove these grimy cesspools of sweaty palm. Easiest way to remove them is by cutting them off
Step 2: Cut All Ties
Cables need to be changed frequently so if your cables are a couple years old just snip them. A new cable set will do wonders for your bike.
Step 3: Loosen Da Bolt
A 3-way Allen thingie is my favorite bike tool ever
Step 4: Remove Barrel Adjuster
Step 5: Unscrew The Screw On The Bottom
Step 6: Remove The Plastic Shell
These shifters are dumb because the function of the shifter is completely dependent on the integrity of the plastic body. This one is clearly cracked. Something must be done
Step 7: Crack Kills
The functionality of your wonderful vintage integrated shift/brake levers.
Step 8: Mix Up Some JB!
This time we'll fix the plastic with some of this awesomesauce. Sorry I don't have a picture of it all gooped up. Didn't wanna get my phone dirty.
Step 9: Now Onto The Big Problem!
The pall seen in this picture near the index finger of the hand model, is frozen. It is not engaging the notches at all and the spring is ineffective.
Step 10: The Only Place To Use WD-40 On A Bicycle
The magic that is WD-40 is mostly bad for most bike things. This is one exception. It will help loosen everything in the shifting mech. Play with it a lot until everything works crisply.
Step 11: Done Dun Dun!
Put it all back together and you've got a nice newly maintained shifter!