Could be either worn pads or a leak. The pistons have to be pushed farther out of the caliper when the pads are worn, which creates a greater volume inside the caliper to hold fluid, thus reducing the level in the master cylinder.this just happened to me the brake fluid level was WaY below low, What would cause this too happen?? could my pads be that worn that it would use alot of fluid to clamp the rotor? or do I have a leak somewhere???