As a retired mechanic certified in brakes I wouldn't recommend topping or filling the brake fluid reservoir.
When the disk calipers are compressed while installing new pads all that fluid is going to go all over the engine bay. If you have all drums brakes (very unlikely unless you're running something from the 70's or older) low fluid is an indication of problems (leaking seals being the most likely).
Unless there is a leak there is sufficient fluid to operate the brakes until the pads or shoes are sufficiently worn to need replacement.
Low brake fluid level calls for immediate inspection for leaks and pad/shoe condition.
We've done so much, for so long, with so little, that we can now do the impossible, overnight, with nothing.
The United States Marines, America's 911 force -- 6531, 3521 '79-'99
Retired mech (auto/aviation), Extra Class HAM, NAUI/PADI OpenWater