Check the O-ring slot for porosity and/or rough spots. Any deep scoring, etc in the slot can keep the O-ring from sealing. Some wheel bearing grease won't hurt, either, to fill any shallow irregularities.
Make sure the O-ring protrudes sufficiently from the slot when installed, too. If it doesn't crush enough, you won't get a good seal.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!