Spongy brakes is always caused by air in the brake lines, but there can be several different causes for this like brake oil seepage from any worn out pipe or component,some times due to master cylinder seals or master cylinder's sleeve itself.
Have you noticed any sort of brake oil reduction in the reservoir?
To bleed the car,fill the reservoir up to the brinks,fix the wrench to the bleeder valve,take a crystal jar,fill it with the brake oil to the half of it,attach a transparent pipe with bleeder valve and submerge the other end into the half filled jar,ask some one to press the brake with a steady pressure on pedal ,now loose the valve a little,air will be released,do it for two to three times until the bubbles of air stop coming out,this way your work is done properly and dont forget to fill the brake oil again in reservoir up to the max mark.
Now feel the brake pedal if still not satisfied,there might be some seepage of oil from a point or might be master cylinder.