Look for wear marks on the plates next to the vanes. Verify cam rollers shift a little when the cold start lever is pulled. Check that the little metal vanes themselves slide in an out of their slots. Lube with Vaseline. It melts when the diesel hits it. Check for plugged delivery valves on the back of the pump. Air should go out but not in as there is a spring in there to keep the pressure in the lines once it is built up. Check the tiny collar on the shaft to see if it is moving smoothly back and forth.
Remember all the pump parts need to stay in the same pump. You can't mix the back end of one pump with the internals of the other. They are machined to be a pair and are at such close tolerances you should have lube on them to get them to go together.
Seals on the front end should not be dry and cracked looking. It should be super clean inside and no sediment or build up showing. Outside needs to be clean to handle it without getting it inside the pump.
Hope that helps.