I've never heard of anyone having a leak on a pump that hasn't been touched but the fact that you are missing a bolt is a pretty good lead on the required fix. You can probably remove another one and match it up with something from Home Depot or Lowes since they tend to use standard metric hardware on these things. Or maybe someone on the board has a junked pump they can pull one from an measure it for you?

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz