I finished the brakes on my 73 Dart a couple weeks ago (BB disc up front, 9" drums rear). It's been sitting for a couple weeks now with the front wheels off, on jackstands (nose up) and now the pedal goes to the floor. I can't find any leaks on the floor or on the cylinders anywhere. What are the chances that this is some air that has just worked it way up from the rear? I didn't bleed the rears because one of the bleeder valves is busted off, and I plan to switch out to larger drums (or better yet discs) in the near future. So I don't want to waste any money on them. The pedal was awesome up to a few days ago when I last checked. Thoughts?