Rear brakes won't bleed

[Whistler]

I have Aerospace 4 piston fronts and a 01 Z28 master cylinder already installed on my car, and still have the stock drums on the rear. I have a complete 01 Z28 rear brake package ready to go, but am getting ready for a dyno challenge this weekend, so I'm trying to get by with the drums temporarily.

Problem is I am getting very little pedal when bleeding, and the front calipers are only just starting to grab as the pedal hits the floor. The front brakes bleed fine, but the rears always seem to have a little bit of air in them, no matter how much I bleed them. I'm thinking the rear cylinders could be shot and letting air in when I try and bleed, but not sure. I guess I'm going to have to add a rear disc conversion to the list of things to do this week..... any suggestions?