Smog pump pointer
I don't know if the 420SEL is that much different from a M103 in a 1991 300TE, but there was a vacuum operated valve in the air pump discharge line that died on mine and, I believe, hastened the demise of my first rebuilt air pump. The warranty on the second rebuild, this time from thebenzbin, specifically mentioned the warranty would not be valid if this valve were defective. It was about $35 and easy to replace.
Again, your model may not have it, but check it out.