While flushing the brake fluid in our 1998 EX we noticed that there is a bit of a black smear around the top of the master cylinder plastic, normally covered by the rim of the cap. Most of it wiped off with a paper towel, but some of it remained. There were also some small bits and almost dust of black junk floating in the filter which we wiped out. That position on the cylinder corresponds pretty closely to the location of the O-ring in the cap, which I believe is 46669-692-003. (On Honda parts sites it looks like a flat round gasket, but for some reason it looked like an O-ring in the cap.) It looked OK, but after nearly 30 years it may be slowly breaking down, Have any of you ever replaced one of these? Take it off with a pick and put the new one on?

Also, what is the trick for removing the cap? It wouldn't budge for me but my son was able to remove it. He is much larger and stronger than I am, so maybe it just takes a lot of force? If there is a gentler method that would be good, because applying lots of force to old plastic is usually a bad idea.

 

Crud builds up on the MC cap, also it can become distorted if installed incorrectly. Tabs get flared out thread can get damaged. Take the cap off and clean it and the reservoir rim. O ring is easily removed with a pick.