1993 Chevy Cheyenne brake fluid leak

Brake light was on, opened the reservoir and the front part was empty, added fluid and tested brakes. Feels like a lot of air got into the brake lines. Checked under the truck for signs of leak, very wet all over due to winter. Might have a leak in rear brake line but no puddles. I tried to free the brake line bleeder on each wheel but they are so rusted they won't break free. What can I do to pinpoint the problem if I can't get the air out of the line