Air pump issue 190E 2.6L

[Doctor Who]

Hi All, I have a 1990 190E 2.6L. Recently, when I start it, I get a noise that sounds like a squeaking belt but goes away within a few seconds. Then it got worse. I figured that maybe the belt was a little loose so I tightened it slightly. Didn't help. The squeak sounded like it was coming from the air pump/idler pulley on the left side of the engine so I found that if I sprayed the air pump pulley with a little WD it went away...for a while. So it apears that I may have an issue with the pulley. To replace it would cost over $400 just for the pulley or I could get a whole remanufactured air pump, including the pulley for about the same. My question is, am I on the right track? Have others had this problem and if I have to replace the pump, how difficult is it?

Thanks for any help you may have.