The Stovebolt Site Forums The Shop Area Engine Shop Dual Master Cylinder Question.

PS- Combination Valve-

I believe many combination valves also include a sensor to indicate when the hydraulic pressure drops, which is an indication of a low fluid levels caused by a leak or failure. Probably wouldn't be hard to wire up an indicator light to the dash or modify the instrument clusters to add an innocuous light for that purpose if you decided not to cut the floor but were worried about fluid levels.

You can use a combination valve with either just a master or a master/booster combo.

Also, an older hotrodder I know used a dual master and different sized lines to the front discs(smaller) and rear drums(larger) on his '40 chevy coupe instead of using a proportiong valve or metering valve. He did use a residual valve to the front but not the back. I would think carefully about that approach before using it since I don't know anything about it beyond having heard of it, and the cost of swapping all the lines versus putting in valves might not be much different.