DISCLAIMER : I've not yet had to take one fully apart but the idea is to push the back plate into the pump housing proper so I'd remove the pulley and use a deep socket against the pump's housing not the shaft, making sure it's not cocking ever so slightly as that'll prevent it from pushing fully home to where you can install the snap ring again .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better