Once you remove the shaft and the 4 cap screws (with little metal blocks on them) the unit still will probably not separate easily.
You will probably have to use a screwdriver and hammer ( cringe) to pound in there to wedge it apart. Its a tight fit plus any varnish buildup kind of sticks it together pretty tight. Once you get it open its pretty easy to disassemble and rebuild. And, if you buy the o-ring kit from Ambac, don't buy the $25 delivery valve spacer separately because there is one included in the o-ring kit. I assume you're trying to check if the plunger guide (dogbone) is broken?