Well we have replaced hundreds of front pump seals I don't think I have ever had one come back. It is a truely simple job... IF you have the tool to pull the pulley. It has to be pulled from the hub not the diameter.
The pump shaft needs to be tight in the front bushing and not have too much end-play; probably an experience related measurement, but we have only thrown away a few pumps over the years even with plenty of used ones available.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician