You using Reliant rear brakes Dave? I had the same problem on my first trike, an XS750 Yam, (in the Motors, 3 wheels section of the site). Tried to use the original Yam master cylinder but it wasn't man enough for the job.
I'd say stick with the matching Reliant unit. Alik Windrush did a piece in 100% Biker on converting the pedal action from horizontal to vertical, but I haven't got any back issues. You could ask on their forum.
Or try Merlin Motorsports or similar.
http://www.merlinmotorsport.com/index.phpThey sell all kinds of cylinders in various bore sizes.