Is it common for these pumps (used on 1997 E420, 1996-97 S420, S500) to develop a leak such that you have to replace the whole assembly? Or is a leak usually caused by an o-ring. I see that there is an o-ring to the reservoir sold in the parts shop.

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)