Originally Posted by Katoom72
Get a repair kit for the master cylinder, even if it's not the culprint it's cheap and there is no harm in refreshing it.
In case this is the culprit though, the new seals will rapidly disintegrate within a month or so, operating in the damaged cylinder surface. These cylinders are colored with a hard paint. When this gets damaged, the seal gets eaten and leaks. The cylinder then needs replacement. I had good luck resleeving this with stainless steel sleeve and going down in diameter. I used lighter KTM bikes' clutch piston. The effect is holding nicely till now.