199 cavalier brake bleed issue

I have a 1999 cavalier. I replaced the rear shoes and right rear wheel cylinder with no problems. When I bleed the system the right side that I replaced the wheel cylinder bleed out fine. The left side I bleed for over an hour and kept getting LOTs of air(manual bleed method) There isn't any leaks and I was carefull to make sure master cylinder was always full. Any ideas on where air could be coming from ABS, master cylinder??