Dual M/C conversion

[Mark Mason <mason@postdiluvian.org>]

I'd like to replace a 3/4" single circuit Girling brake master
cylinder with a dual circuit one.  It sounds like I should just use a
double with half the surface area of the single, which works out to
1/2".  Is it really that simple?

The car in question is a 1965 Morgan 4/4.