check the whole brake system for leaks, start up top, around the master cylinder, then follow the lines down, check the front calipers, follow the line[s] to the rear, last, check the rear wheel cylinders. any leaks? none? block the wheels, if your on the ground. have someone press the brake pedal and hold it, see anything leaking, front or back? does the pedal continue to drop?......also, the master cylinder could be leaking by internally, or back into the booster. low fluid does not mean there is a leak, brake pads and shoes wear, as they wear the fluid level drops[to fill behind the caliper psiton]