Somethig to keep in mind, is that the master cylinder is actually, two master cylinders in one. One half for the fronts and one half for the rears. If you get a leak in the rear, the front part of the resivoir goes down and vice versa. If the master messes up, it is often the fronts or the rears that give trouble, rarley both. And when half of the master starts to fail, the brake trouble light will come on, because the two sides make (radically) unequal pressure (there is a pressure switch for this purpose).
I´ve had the brakes seep (leak) at the wheel cylinder, but also at various fittings in the brake lines.
If the resivoir has run dry, it can sometimes be problematic to get all the air out of the master cylinder, but again the brake light will come on, because of the uneven pressures.
If there is air in the system, the brake light often won´t come on, until the air problem gets really bad.
You can often hear the vacuum booster hissing (when the brake is lightly applied) when there is a diaphram leak.
ABS?