Are you using a power bleeder or the "pump the pedal" method?

I have had problems in the past getting rebuilt brake systems bled, I switched to a power bleed setup and have not had a problem since. It takes about 1/10 the time and works perfectly.

One some models the amount of fluid held in the master cylinder for the rear brake circut is very small, in the W124 two or three pumps will exhaust the fluid supply. Make sure that the rear brake circut has an ample fluid supply.