This car did sit for probably 6 months before I bought it due to engine going bad. I was helping someone move and they noticed my rear discs we completely rusted over. They pretty much looked as if the pads haven't touched the disks. I took the rear brakes apart, cleaned everything and was going to replace the pads when I noticed the pads were all perfectly fine. I read this thread Only two brakes work!
and it is similar to my situation but its both rears that don't work and both fronts are doing all the braking. I hope it isn't a master cylinder, but after reading it may be. I am looking to see what I can do to check this before thinking about replacing it. Once pedal is pressed initially and the valves are opened, the brake pedal does not sink to the floor pushing fluid out. There was some fluid in calipers when I took the lines off and cleaned the calipers and VERY little fluid would come out of the rear flexible section of the line connecting from the metal line to the caliper. I had to twist the line around to get any fluid to drip out and gravity bleeding didn't work. If there was air in the line, it should push out with depressing the pedal and the pedal does not depress after opening valve. Sorry I am being redundant, but I am just trying to cover everything.