1989 300E brake issue

[JasonDTM]

Greetings,

Encountered a problem with the ol 300E awhile back, brake pedal would come and go as it sees fit, would come back with pumping.

Replaced

Master Cyl <--- New
ABS Pump <--- Used take off
Rear Brake Lines

Have bled the system key on engine off with a power bleeder, no air in the lines but car will only stop on the front braking circuit.

ABS light never came on with the original pump while the engine was running, comes on with the replacement pump.

Short of rebuilding the calipers I am at a loss, any help would go a long way.