In my experience that seal hardens up and causes a leak. It's not too hard to replace it. I did a short write up on the procedure in a diffrent thread.

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine