New calipers on, but trouble bleeding brakes

[merrim19]

Vehicle: 1986 300E 228K

A broken rear axle let me to change out the rear rotors and pads. Due to whatever reasons the calipers decided to freeze up, so I figured I might as well replace them too. Everything bolted on fine, but I have air in my rear lines. Front brakes work just fine, but rears arent grabbing the rotors at all.

So air in the lines is causing the calipers not to grab, I must bleed all brakes. No problem, done this many times on other cars. Manual pumping the brake and releasing the preasure out of the nut on the caliper.

The preasure does push air out, but air must be getting in somewhere because I have been purging the lines systematically for a long time. Same amount of air and a little fluid comes out.

Possible problem with the master cylider? But then why do my front brakes keep working. I'm hoping I'm just missing something obvious, like this peticular car must by running in order to purge the breaks, or that it can not be done manualy, without a powered system.

Any ideas? I'm stumped and winter in rolling in.