If you're actually losing fluid, it's going somewhere. Do not guess on this, find it, as everyone else says.
Since it's a dual reservoir, start by finding which end is losing it. The small reservoir goes to the rear brakes, the large to the front. Check rear slave cylinders if it's the rear that's going low. If it's losing both equally, look to the master cylinder itself, and check for fluid in the booster from a bad rear seal. If the brakes are behaving badly but no fluid is lost, it probably is an internal master cylinder leak from worn piston cups (new MC called for).
No guesswork here. If you see fluid escaping, be specific where it is.