I had a similar problem with my 2000 Sportage (also manual). I didn't notice the low brake fluid level until the low fluid light on the dashboard flashed on a few times while driving. There was no sign of leakage on the ground where I parked, but the reservoir sure was low. I topped off the brake fluid, but didn't drive the car for a few days. The next thing I know there's a substantial puddle of brake fluid under the car. It had been parked on a slight forward, downhill pitch and the fluid was coming out of the front of the master cylinder, no doubt about it. When I removed the master cylinder, I noticed that the bore was worn or eroded only at the point where the front piston contacts the bore so that it leaked when it was just sitting there, but not when you applied brakes. I installed a new master cylinder ($150 from the dealer) that fixed the problem completely.