You have probably figured out that you can take the caps off the ends of the rail and clean it out. It is just a hollow tube. If you turn the engine over for 30 secs with the starter with the coil wire disconnectd, you can verify that all the lobes are getting their oil through the little holes in the rail.
Kind of messy, but worth doing to check.
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