To go back to your original description and question, you are obviously losing pressure slowly. The longer the car sits, the more pressure it loses. Now that I think about it, if there's not enough fuel pressure, then the cold start valve won't do much good.

AllDataDIY suggests reconditioning the pressure regulator in the fuel distributor with new o-rings if there is a pressure drop below 2.5 bar after 30 minutes of sitting.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe