Neither of my '97 328s seems to bleed properly, and both of them have chronic "Low Coolant" messages in the computer.
I pulled the radiator out of one of them on my way in to replace the original water pump, and I noticed a rattle in the overflow reservoir.
I shook out a couple of pieces of broken plastic. That can't be good.
Ordered a new reservoir. Took my Dremel tool and split open the old one to find out what failed inside.
I still don't fully understand how water can flow out of the bleed screw hole when the reservoir level is 2-3 inches lower than that hole, but it must have to do with the stream of hot water coming in through the little 1/4" hose. That hot water blast also seems to attack the plastic.
However the bleed works, I am pretty sure that having a chunk missing from the split tube that goes from the bleed screw down to the bottom of the reservoir will have a deleterious effect on the bleeding procedure.
By the way, this failure was not visible through the fill hole. I rotated that tube around so that I could photograph the failed section. It was facing the side of the tank originally.