It looks like I've got to replace the AIR Pump on my '92 300E 4Matic. I think I'll order a rebuilt pump and try installing it myself. I've been told I should probably also replace the Check valve and Air injection shut-off valve as well. How difficult a job will this be? Any tips/pointers? Any manual or source online that might be helpful with detailing this procedure? Where do the valves actually reside? Any help would be much appreciated!