The master cylinder is probably junk. If you had a good brake guy, he could clean up the surface inside and just use a rebuild kit. Myself I would buy a new m/c and start over. I dislike what is normally the quota flow. There are several after market options. Nissin and a few others are available, but pricey and not a whole kit. I threw away the braktec`s on my Montesa 301 as I had been down this path before ( I still have a nissin m/c to an ajp caliper as a spare} I run the large ajp m/c to the TRS gold front caliper. Strong and dependable. You don`t want to ask about my clutch. LOL