That seems to be a common fail point on the T-Series, my old 420GSi leaked there too - not out but in! In other words, it drew air in which caused air-locks and sneakily emptied the cooling system resulting in HGF and the head had already been skimmed within a micron of its life. After the HGF it was more banana shaped than a banana shaped thing!
I'd suggest when you fit the new seal, a smear of silicone grease (or red rubber grease) on the seal and also dress the block/pump housing with emery paper to remove any oxidisation so it's as near perfectly smooth as you can get it. This will all help the seal find its water-tight position by allowing it to move very slightly under its own rubbery properties.
You probably knew this already but it might help someone else with a similar problem.