you are getting air in your brake line
when you bleed it, stick about two feet of clear plastic hose that fits snug over your bleeder (get from hardware store), then put the other end of that hose in some brake fluid thats sitting in some kind of resevior. THis keeps the end of the hose underwater so no air can get in when the brakes are pumped, take apart your master cylinder and check it and pump your brake or push it with a socketdriver if your lever is broken untill no more air bubbles are in the hose coming out of the bleeder. By now the brake should be very firm and hard to push in. Tighten the bleeder, fill her up and you should be alright, if not then who knows