White Post has done two master cylinders for me over a 20 yr period, and each time I asked if they could/would do it in stainless; each time, they replied they ONLY use Brass. I'm sure there's some reason for that; my suspicion is that it's a bit softer than SS, therefore easier to hone to the correct diameter (but I'm only guessing). The only other aspect I can imagine is possibly thermal expansion coefficient differences but both have been used successfully in many MC, WC, and calipers...