If the rear wheel cylinders were raise above the master cylinder when you took the tires off, air could have entered the system at the wheel cylinders and all the fluid could run back to the master cylinder. If this did happen, one or both wheel cylinders are bad. If there is no sign of a over fill at the master cylinder, start following lines as already suggested till you find the leak. In this case, all the dust and dirt will work in your favor.
Joe
