You've got the basics right. Either the pump is not supplying pressure, or the valve is not passing it on the the "legs." Between the two, I would think the pump is the more likely failure.

All this based on rather limited experience with my S500 Coupe, which has basically the same setup.

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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