From;
MBMBrakeBoosters.com under the Tech heading, topic; Which master cylinder is for me;
DISC / DISC
A four wheel disc brake master cylinder is designed to supply more fluid pressure and volume to the rear disc brakes than the disc / drum master does. This is achieved through an internal piston re design. The piston that feeds the rear brakes on a disc / drum master will run out of stroke, limiting the amount of fluid pressure and volume that may be supplied to the rear disc brakes. The four wheel disc master re design delivers the extra needed volume and pressure to the rear allowing your rear disc brakes to function properly. If you attempt to use a disc/drum master on a four wheel disc system you will get poor rear brake function and experience a spongy brake pedal with a long pedal travel.
This is from SpeedwayMotors.com;
Universal Power/Manual Master Cylinder, 1 Inch Bore, Corvette-Style
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Item # 91031445
$59.99Quantity:
In Stock
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Brake Master Cylinders
Specs
Centerline of Holes:
3.38
Material Type:
Steel
Overall Height:
4.75
Overall Width:
4.50
Sold in Quantity:
Each
Finish:
Natural
Mounted Length:
8.00
Bore Size:
1.00
Feed:
Dual
Outlet Fitting:
3/8"-24
Garage Sale:
No
Notes
Pushrod to mounting flange is 5-1/2". Two 3/8"-24 IFF outlet ports on both sides of the cylinder. Plugs are included for the unused ports. Works with both manual or power brake setups.
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Q & A
These high-tech corvette-style master cylinders were designed using OE specifications and 100% tested. They have a 1" bore and are suited for disc/drum combinations. Each master cylinder is ported on both the left and right sides, allowing for installation in many different applications. This exclusive design features a mating flange that works with applications requiring 3-1/8” through 3-3/8” bolt patterns. These master cylinders will work with manual or power brake systems and can be quickly changed with the included adapter pin to plug the deep well. The wide body and low profile gives this master cylinder the clearance you need when installing in tight areas and yet it maintains a large volume of fluid necessary for 2 and 4 wheel disc brake applications.
1" bore
3/8"-24 IFF outlet ports on both sides
Plugs are included for the unused ports
Mount spacing is 3-3/8" on center
Works with power or manual brake systems
Compact, for installation under floor or on firewall
Suited for either disc/disc or disc/drum combinations
Stroke of this master cylinder is approximatlely 1-1/16"
This is from PirateJack.net
Can I use a disc/drum master for four wheel disc brakes?
Usually not. For 4 wheel disc brakes to function properly you will need a master that has a longer stroke.