After trying to bleed the rear brakes using every method known to man, I determined the master is the problem, RATS!
Okay BMW brake experts, put on your thinking caps...
So here's the question; should the new piston, with the cup shaped seal, have holes in the front like the old piston?
I'm thinking my problem is that with the new piston (no holes) I'm not getting brake fluid flow from the master reservoir to the brake caliper when I release the brake piston and it returns to the starting position?
This is the old piston I took out, note the hole in the seal grove
This hole goes through to the front of the piston, there are two of them.
This is the old seal, note that it is not cup shaped but rather it is rounded like an o-ring...
And this is the new piston with a cup shaped seal and no holes under the front seal.