My wife recently got a used '04 CRF230f that is in excellent shape with the exception of the front brake master cylinder.
The lever (plunger) doesn't come all the way back out of the cylinder and the brakes have very little feel to them - they're more like a light switch - on or off.
If I'm careful I can ride it ok, but she doesn't have a lot of experience and tends to lock the front wheel - not good!
I've never worked on master cylinders and wondering what I need to do to service it or check for damage. I would imagine that some part just needs cleaning or replaced but I have no idea how to tear these apart.
Any advice?
Thanks!
Mike

I've rebuilt a lot of these guys on older Hondas, and usually there is a little metal wire split ring that holds the rubber boot on (where the lever itself presses on the end of the piston.) After you get the boot off, there is an internal circlip that holds the piston assembly in the master body. It requires a set of long nose snap ring pliers to remove, usually (although I have managed it with 2 small awls, but it wasn't easy!) Depending on how stuck the piston assembly is, you can usually get it out by blowing compressed air in the fluid outlet hole (do it in a box with a rag covering the end of the master body, and keep your hand out of the way, the piston sometimes comes out with a great deal of force.) If it is really stuck in there, I have fitted a grease zerk in the fluid outlet hole and used a grease gun to "hydraulically) remove the piston. A rebuild kit should be available from honda that includes the rubber cups, springs, clips, etc that need replaced, (although if it is just sticking you may inspect the parts and determine they just need cleaned up) and you need to clean out the bore of the master and make sure that there isn't too much pitting (I've "honed" the inside of them using scotch brite pad to good effect.) You also have to make sure that the tiny hole(s) that go from the piston bore into the fluid resevoir are cleaned out and open (be careful, you don't want to enlarge them, just clean them out.) You can find an exploded view diagram of the master to see how it comes apart/goes together on www.crotchrocket.com by going to the "OEM parts" lookup for your specific bike and looking at the diagram for the front brake master.

Now, all that being said, if you don't feel comfortable with all this, don't experiment! There isn't much more critical on a motorcycle than brakes, and doubly so if your "boss" is going to be riding it. If you have any doubts about your ability to do this, please get help or let a professional handle it. If you are half way handy with tools and do most of your own maintenance, though, you should be able to handle it. Good luck!

Oh, and by the way, if the brakes are sticking on, you probably have a dirty caliper as well, and it should be cleaned/rebuilt as well. It is eaiser, as there typically isn't a clip holding the piston(s) in the caliper, and the compressed air or grease + zerk will work to remove the piston in it as well.

Thanks for the detailed write-up! I just looked at the exploded view and it looks like the part that might be causing the issue is about $60 + shipping to replace. I just found a complete used MC for that model on ebay for $45... I'd love to just pull some parts out and clean them, but since I've never worked on a master cylinder before and don't have some of the tools you mentioned, it'll probably be cheaper and easier to replace the entire assembly and keep the old one for parts.