Did you full the unit with fluid before fitting it? It's not necessary but does help. I'm not sure of the configuration of the e36 but you will need to active the abs pump to purge the air out of it. The e46 for example can be activated with inpa. Depending on the system type some will have a relay. Crossing the relay will cycle the pump.

Get someone to activate the pump and while running open the bleed nipple for a few seconds then shut the nipple and stop the pump. It'll take a good few cycles to get it spot on.