A common issue on all bikes, nothing to do with the master cylinder and many a person has purchased a master cylinder kit and/or a caliper seal kit thinking they were sticking. when all was required was a little bit of lubrication..
I covered it in this thread http://foc-u.co.uk/index.php/topic,14903...#msg168810
Use a little WD40 to see if it is the problem and if it is use red rubber grease, but any grease will fix it.
I have seen a number of bike mechanics genuinely caught out by it
The movement is small but enough to take the feel from the brake lever and make it feel like something more serious
I covered it in this thread http://foc-u.co.uk/index.php/topic,14903...#msg168810
Use a little WD40 to see if it is the problem and if it is use red rubber grease, but any grease will fix it.
I have seen a number of bike mechanics genuinely caught out by it
The movement is small but enough to take the feel from the brake lever and make it feel like something more serious