Bill - From my reading on the YF's that "check disc" is just another term for a "check ball". I'm pretty sure Jon "carbking" will confirm.
My own experience with disassembling a YF2100S ran into the same problem with the diaphragm housing/assembly not wanting to come out. Turns out it was stuck together by the diaphram itself after having been assembled and left for an eternity.
I ended up having to gently pry on the side lip of the housing and the casting with a very thin flat tip screwdriver wrapped with a layer or 2 of electrician tape, to break it loose. Gentle repeated persuasion attempts and it should release.
