The outer diaphragm ring sticks to the pump housing (it is a seal ring). The middle portion connects to the rack. You don't need to remove anything else, just disconnect the outer ring of the diaphragm.

There are shims and spring inside, so be careful when you pull the diaphragm out.

When you put in a new diaphragm, you need to measure the compensation path and set it correctly for the engine by removing or adding shims. The procedure is described in the FSM.

Most air leakages occur around the axle of the poppet cam, by the way.

@ moon161

MB USA has put the FSM for the W123, W124 and W201 online for free:
http://www.startekinfo.com/StarTek/doclist.jsp