Here is another link I just came across.

Helpful Secondary air pump tip - MBWorld.org Forums

this thread is particularly cool in that has a link in the first post to a rebulder (who markets his service thru EBAY) that charges ONLY $95 to rebuild these

Mercedes Benz Secondary Air Pump Repair R230 W220 W215 W211 W219 W203 AMG | eBay

Cheers,
Chris