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Well, discovered one of my rear calipers was missing both the adjuster hole plug and also the adjuster. Now i am having a hard time getting the new adjuster to seat in all the way. I can only get it in far enough to expose a few of the threads that the plug uses.
Also, when i turn the adjuster, I can get the piston to extend, but not retract. Shouldn't it move it both ways?
I have complete kits.
If the cap was off and the adjuster was missing, I'd say there's probably a good deal of dirt built up in there. The first thing I would do is take a high pressure air hose to it and blow it out. You may want to try brake cleaner and then air. Solvent based cleaners (even brake cleaner) can swell seals and damage rubber. It's highly advisable to air it out quickly if you use this method. Do not use carb cleaner as it will quickly destroy your seals. Quick shots of brake cleaner followed up with air to evaporate it. Best is just air if you can get it in there.
Do what the Cap'n suggests and apply a slight amount of pressure as you attempt to turn the piston in. Remember, that inside adjuster is gear driven and it turns "counter-clockwise" to pull it in.
If you decide to rebuild, follow the link below for the calipers. In the first post you'll see a "Do-It-Yourself" link. Read along and PM me with any questions.