Losing brake fluid with no leak
It's a sealed system, so it's got to be going somewhere, leaking somewhere (or boiling off from severe braking operations over and over). Since you likely haven't experienced extreme braking, it must be leaking somewhere.
You will have to trace all your brake lines from the master cylinder to each wheel to check for any sign of leaking. Front calipers are easy to spot leaks when they leak. If you have drum brakes on the rear, you may need to pull off the drum to check the wheel brake cylinders. Peel back the rubber boot on each end of the wheel cylinder and look for any fluid leaks there.
Another place to check is at the back of the master cylinder. If a seal is bad there, it has been known to leak fluid inside the car, under the dash. Feel around the carpet under and behind the brake pedal for any wetness. If you find brake fluid there, you will need the master cylinder replaced or rebuilt.
Hope you find the problem. Don't forget to check the ABS system and brake lines, too.
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