Mercedes Benz repair: Bleeding breaks Mercedes 190D, mercedes 190d, hand vacuum pump

Question
Having difficulty bleeding the rear breaks on a 1984 Mercedes 190D. All we seem to pull through the rear bleeder valves is air. Have used both hand vacuum pump bleeding as well as traditional two man brake bleeding. Disconnected rear brake line at master cylinder. Able to pull vacuum and have it hold so no leaks in rear break lines/calipers. (Note: No problem bleeding front brakes.)

Replaced master cylinder with brand new unit. Same result. All we pull or pump is air. Is there a special trick to bleed rear brakes on this vehicle? Thanks!

Answer
Need two people for this.
Fill master full.
Open one front bleeder and one rear bleeder.
Have helper step to floor and hold pedal down, until you close both bleeders.
Keep doing this until you have fluid to rear wheels, then bleed rear only.
Keep master full at all times.