If it's doing its job (i.e. holding pressure) then I wouldn't worry about it. I'm sure some purists here will scream otherwise.

The OEM part is cheap, so if you REALLY feel like "keeping it real", then pick one up here at FastLane for insurance...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car