Not necessarily so! I'm using a disc/drum master cylinder on an all disc setup now. The key here is you need to understand hydraulic brake systems thoroughly before making changes. You need to know that the master cylinder you're using will deliver enough fluid volume to apply the brakes, and how the stroke and bore diameters affect braking power before making changes. I haven't found any problems with stroke and volume, but bore diameter has an effect on pressure. A good reference source is the tech section at www.mpbrakes.com
. There are explanations there on pedal ratio effects and bore size.
The reservoir size is usually larger on disc brakes because as the pad wears the piston doesn't move back as far, causeing the fluid level to gradually get lower. US manufacturers decided to make sure there was enough fluid in the front brake reservoir to last the life of the pads and still stop the car even if you never added fluid. The drum brake (rear) reservoir is small because drum brakes have return springs -- the fluid is always returned to the master cylinder no matter what the wear is on the shoes. I check my brake fluid often, so I never worry about reservoir size. Pad wear is gradual unless something is wrong, so there's no real concern if you check the fluid level 3-4 times a year.
I have to add that my current master cylinder is a newer type (late 90s Ford Ranger) that has a single reservoir for front and rear brakes. Doesn't matter much about pad wear -- the same reservoir is affected by both ends of the car.