There is a design flaw in the siphon jet, did you already address the o-ring defect? If you have the symptom of the right side dropping to zero when the left side still has say 17-20L that's typical symptom for leaking o-ring on the siphon jet. The system doesn't need to be replaced usually just the faulty o-ring

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)