M10 can have two thread pitches: 1.5 and 1.25. Into iron or aluminum, the 1.5 pitch is much more likely. If your bolt is 10 x 1.5 and it doesn't go, you might have to use a bottoming/chasing tap to clean the hole.
A little calculation shows that piston configuration is the equivalent to 4 x 40mm. Fairly small, so I estimate that 5/8" or 11/16" master would be adequate. That, of course, is also dependent on the rear master cylinder size.