I lost the nerve to rebuild my smog pump and went with a rebuilt for $225 from a local vendor. I had incorrectly concluded that the pulley bearing was not replaceable. This thread shows you how to do it.
http://www.peachparts.com/shopforum/showthread.php?t=168879
The pulley in the oven, bearing in the freezer trick is one to remember. Not being able to cleanly press in the new bearing was one of my fears. Curious as to what brand bearing the shop used in the rebuilt, I popped off the clutch and sure enough it was the Chinese bearing that everyone is referring to. It is the larger one in the photo. The other bearing was supposed to be the inner bearing but the shop guy said it was too large.
Now most of the online sellers of rebuilt pumps require you to also change these 2 valves, the check valve closest to the engine and the diverter valve closet to the pump, in order for the warranty to be valid:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=22O14A4OD22Q03CZP0&year=1991&make=MB&model=300-CE-002&category=B&part=Air+Inject+Check+Valve
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=22O14A4OD22Q03CZP0&year=1991&make=MB&model=300-CE-002&category=B&part=Diverter+Valve
I don't know how they would enforce this but having changed my check valve ( as low as $15 if you shop around, otherwise ~$50), I can see the reasoning. The check valve is a simple one way rubber flapper valve with what appears to be a heat sensitive coiled spring shut off valve on the engine side. The engine side of mine was covered in soot. If the valve got jammed open than all this stuff would eventually make its way back to the pump. My diverter valve seemed relatively clean (plus it was $50+ for a new one) so I reinstalled it. The two rubber hoses were still in excellent shape after 150m, but I had new ones so I used those (~$20 for both).
glenmore
1991 300CE
2000 C280
1990 LS400