Don't top off the fluid in the master cylinders! Everybody seems to do this. Somewhere in the middle is where you want to be.
Quite possible. I just noticed that the OP is having problems with both front and rear, which is a strong hint that there's a faulty technique involved in the re-assembly.
The rear reservoir I fill only to just above the minimum line. Yes, there is some pad wear that will draw it down over time, but on the rear, it's so minimal after the pads are fully seated the first time, that it's typically not worrying about over the course of a year.
If the front brake has a remote reservoir, same principle applies. With the older built-in coffin reservoir like mine has, it's trickier because most often, they are not level. In this case, filling so the level divides the sight glass is a big mistake. What works for me is to remove the the rubber diaphragm wedge thingie and twist the steering so that the fluid is piled up the most extreme in one corner. Fill until it's just short of overflowing, the straighten the steering out to make the coffin closest to level. Insert the rubber thingie. If it doesn't overflow and the fluid still covers the bottom holes leading to the cylinder, you're good.
Too, I've found that with this old bike, fresh fluid was turning black rather quickly. Using a syringe, I removed the black stuff contaminated fluid from the reservoir and replaced with fresh. Several repetitions later - very little contamination remains.