I am getting older, and keep thinking "what if...." so it seems sensible to change my single line brakes to a dual (or rather tandem) system

Now I use a 0.7 single line master. And here is where I get stuck. If using a dual line, do you reduce the bore so the area of the 2 pistons adds up to the same as the single line (that would make mine a pair of 0.5 bore pistons!) BUT it seems the same bore is used single or dual?? Anybody able to explain?

(car is a spitfire, bigger/vented GT6 brakes front, MGF rear discs/callipers) There is a GT6 dual master available, but its bore is even larger, and from experience a larger bore feels dreadful. Spitfire duals available, but may be too small? However, there is a TR6 one which looks the right shape apart from the mounting flange is incorrect, but not difficult to overcome.

Help with the bore size would be greatly appreciated..

(edit to clarify I will swap to a tandem master cylinder, not twin)

[Edited on 23/3/19 by cliftyhanger]

Was the larger bore cylinder used with a servo which would reduce the amount of pedal pressure needed? Might be a worthwhile upgrade ?
I suspect that a 2 piston cylinder would produce the same line pressure as a single piston item given the same pedal pressure. Been 45 years since I calculated line pressures but I doubt things have changed. Replumbing would be easy

The GT6 initially used single line brakes and teh 0.7 MC. At some point near the end it went tandem, and added a servo. On my car, having squeezed an ST170 and all the gubbins in, there is no space for the (remote) servo they used.

My worry is the bore. Too large and it will all feel so wooden, which is not a good thing. But I "think" if I use the same bore as the single line on a tandem, it may feel the same. I can't imagine a 0.5 bore MC being used on any cars.

A spitfire non-servo tandem cylinder is 0.7" bore, which will give you the same pedal effort you currently have.
The servo version is larger bored and will give a rock hard pedal that will be ineffective in use without a servo.

Going to dual circuit also will reduce your mot (if required) handbrake requirement from 25% to 16%, i think the mgf h/b calipers would be marginal for 25% effort.

Dave

if I remember correctly...
although the later gt6's with tandem systems used a servo, (which only acted on the front brakes)
there was a factory option for dual circuit brakes for the earlier cars. non servo

https://rimmerbros.com/Item--i-213690

managed, with some digging, to find the GT6 tandem is still a .75 bore.
The spit one looks interesting, and I know somebody who owns a spit with that fitted, so I will blag a drive.

Re the MGF handrake callipers, they are WAY better than the std drums, and unlike triumph rear brakes, always pass the MoT! (have done allsorts in the past to get the handbrake to pass, on many many small chassis Triumphs)

Thanks all....