spongy brakes

If you can't keep brake fluid in the reservoir it has to be going some where. Pull off the vacuum hose to the brake booster it should be dry as a bone, If it is wet or even moist the master cyl. is leaking fluid. and the vacuum diaphragm in the booster has been damaged. You will probably need a master cyl. and a vacuum booster. If the hose is dry, get under the car and check behind the brakes at each wheel for fluid leaking at the calipers or rear wheel cyls. just because there is nothing on the ground doesn't mean there are no leaks. And if fluid has been leaking out there might have been air sucked in.

There is a presence of air in your brake system check out this information http://www.cartechhome.com/2008/12/car-brake-pedal-feels-spongy-while.html