Steve . . .
When I got my GT about 10 years ago it came with a dual circuit brake master cylinder out of a TR6. Trouble was that it is much longer than the original single-circuit master cylinder, with the result that I couldn't adjust the pedal assembly far enough to accommodate my long legs. So, I changed out the dual circuit TR6 MC for a single circuit one that was identical to what originally came in the GT. If you can live with the loss of adjustment depth, the TR6 MC might do the job for you.
Ron McLeod
Marcos Mantis P8-5858
Marcos GT 3V-5642