these pumps are made by essex industries, their spec material was viton, ulsd makes it expand. The national seal 350435 is the replacement for ulsd as we discovered and passed the info on to essex. If the rotor was metal and the shaft had an induction hardened shaft, they would offer a much longer service life. We sold some that have lasted without issue for 3-4 years, some of them didn't last 2 weeks. I abandoned the faulty design over 2 years ago, since essex didn't want to further improve their product. We also incorporated a weep hole in ours, in case the seal failed it would simply leak to the outside, saving the lower bearing and the motor. It was a futile attemp as the shaft was so soft that seal quickly ate into them causing the leak to begin with.