Real auto parts stores have catalogs that list casting numbers. There should be a casting number on the bottom of the master cylinder. You can probably see it with a mirror. Write down the number, note the side the lines are connected to, and measure the diameter if it is off or call Duff and ask them what the bore is. A good counter person at the auto parts store can match it up for you.
Buy a new one. Some repair manuals say that honing a cylinder reduces life. It allows the piston and seals to move around more and wears out the seals sooner. I have never owned a rebuilt master cylinder. Flush your brake system before you take off the old MC.
