A lot of the old cars had very large wheel cylinders. Its possible the master cylinders had enough capacity to accommodate the disc brake calipers fluid requirement until the pads wear significantly, brake lining wore off the brake shoes back in the day as well, resulting in lower master cylinder fluid levels even in the old drum brakes. If a person keeps up with the fluid level, I don't see an issue. Don't you keep the fluid level up on your modern disc brake cars? I find it hard to believe any real car guy wouldn't check and refill the master cylinder between brake jobs at least a few times.

It really is a question of capacity. The master cylinder has to have enough capacity to accommodate the fluid requirements of the brake system under the braking load, a little extra reserve make us all feel better. This is much easier when everything works as it should, but when things go a muck, systems on the fringe of function get scary. Sometimes mixing the design function of parts of a system (like swapping disc brake calipers where drum brake wheel cylinders used to be) puts things on the fringes of function. I suspect the major question here is, how close to the minimum reserve brake fluid capacity are you willing to live by?

I suspect the new brake systems are closer to the minimum then the old stuff was (as originally designed), in the old days most stuff was overbuilt. These days, the bean counters are in charge, now things are designed to get by with the least possible over build.

A comparison of the fluid capacity between the two wheel cylinders and the two calipers, and the capacity of the master cylinder might revel some interesting results. If the rear brakes have been updated, the capacity differences of the rear wheel cylinders also needs to be accounted for. Without real, factual, numbers, its all just speculation. Gene