I've replaced all the O-rings but it leaks. @davidmash: how did you know your seal was leaking? Was it dripping down the front of the engine? If so, it's hard for me to understand how seating the rear shaft seal deeper stopped the leaking that must get past the cover o-ring.
Mine is leaking badly between the halves when under pressure, but if anything I'd think that means the rear seal is too deep (but it's flush).
UPDATE: I figured out the 2 o-rings I put on the center piece were too small (I ordered the wrong OD and had to stretch them an inch to put them on). I measured the originals and got 2mm cs x 62mm ID, which agrees with what someone else posted for a later car w/ same pump (mine is an '85 300TD).
The cover O-ring btw seems to be 2mm x 61mm. I think you can get that one from the dealer but someone complained about the quality (and I think it's one of those superceded part numbers, not necessarily the same as the original part).