If the Mercedes dealer dose not use the 5/16" hose it is likely they simply have a large roll of the 7mm Hose (I thought it was 7.5mm) that they cut what they need from.

I have has zero issues with the 5/16" Hose. Perhaps you could use the 5/16" house until you find a source of the Hose you want.

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84 300D, 82 Volvo 244Gl Diesel