National Parts Depot sells a master cylinder for a 4-wheel disc setup, presumably with the lighter residual pressure valves that are appropriate. I'm not sure about the bore size, but I think it's actually 1". I just bought one, but haven't had a chance to install it yet. Might be a place to start looking - it's a Wagner master cylinder and NPD's usually willing to give out the manufacturer's part number if you ask for it...
Thanks for the reply. What calipers are you running front and rear? And what MC do you have now? I have 99-04 GT fronts and 95 GT rears with a 1.0625" MC and manual pedal. I'm not sure I could lock up the tires if I wanted to.
I have SSBC brakes all around. In the front, they're the same (approximately) as the original Kelsey Hayes 4-piston brakes that came stock on 65-6 Mustangs with an 11" rotor. The rear has a 10.5" rotor with single piston calipers that SSBC scavenged from an ~'88 Thunderbird Turbo Coupe. The M/C I have now is the stock '67 dual-bowl master cylinder (my car is a '65 that originally had the single-reservoir version), also a 1" bore I believe. Although I'm sure that the '67 has the incorrect residual pressure valves, at least for the rear, it hasn't caused any problems that I can identify.
I open track the car and have found that the brakes are adequate to a point. They slow the car well (yes, a determined poke at the pedal locks the tires), but heat up after extended use and start to get a little scary. They'd be more than adequate for the street, but are marginal for track use. Hope that helps.