1989 300E brake issue
 

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.

Does this mean you sourced the ABS at a pick-n-pull?

If the MC is definitely working on both circuits (usu. verified with a bench bleed upon installation), then I would go ahead and replace/rebuild the rear calipers. 29 in car years is 145 in human years.

Brake booster shot ..Sit in the car no engine running push the brake pedal till its on the floor and hold it down .Now start the engine and hold your foot on the pedal it should lift your foot up if the booster is working .If it wont bring it up to normal level replace brake booster Or you have a vewry bad vacuum leak .