It's very unlikely to be a problem with brew pressure, because if you're getting those kinds of flowrates you probably don't have enough resistance to even reach 9 barg (I'd have to see the pump curve but I don't think a vibe pump will do 420 mL/min at 9 barg).
Flow and flow resistance drive the pressure, it's NOT the other way around.
Flow and flow resistance drive the pressure, it's NOT the other way around.

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