^Exactly.
To my knowledge (although never worked with the type that bleeds ABS through the rest of the system) you're supposed to keep the MC "full of fluid". Meaning not to let it get below the minimum line on the reservoir. You might be able to let it get a tad below, fill it back up, and be fine, but too low and obviously you're letting air into the system, either as you pump the pedal with the cap off, or once you open the cap to add fluid.