Brake cylinder size 1800S

[Grantura SWE]

What size (diameter) is the brake master cylinder of a 1800S (MkIV)?
I'm thinking of upgrading to dual circuit system, or even replacing the entire pedal box to a "modern" one. So I would need to know the diameter in order to find suitable replacement parts.
Thanks!

[Astacus]

Girling 0.7" on my Vixen S1 and I think its identical

[Grantura SWE]

Yes as far as I know Vixen and mkIV share the same parts (except engine and gearbox).
Thanks!