More than likely if you have leakage at the wheel and you found your master cylinder needing refilled, it was leaking before you filled up the master. Master cylinders don't just "need" brake fluid. It is (for the most part) a closed system. The extra fluid in the reservoir is there so as the pads wear on disk brakes, the extra fluid will fill up the caliper and everything can continue to function. There should be enough fluid in the reservoir to compensate for what is needed in the caliper as the brake pads wear out. If you've found it is below this, it is most likely due to the leak at the caliper or wheel cylinder and not because you refilled the master cylinder.