How do I take a Bendix disc brake booster apart?

I managed to separate the housing halves, remove the larger hex nut, the metal dish and first rubber diaphragm underneath. I also took out the push-rod (for the master cylinder).

Now I'm stuck: there is a black dish like separator between the two halves of the booster (seems to be a phenolic resin) I can't get out.
How do you remove it? I can move the cylinder in the center back and forth. Should the phenolic separator just fall out? There is quite a bit of rust in the booster - maybe it is rusted to the housing?

It should be part B in this drawing:



any ideas? Thanks