Strange thing is in all the Giss systems I have done, the rear balance seal has not had the dreaded bacterial degradation. The only reason I can think of is that any oil that has been added through the transfer port has only reached the front compression seal. Older type polyurethanes degrade and break up especially through moisture and humidity. When I sold it in South Africa in the 70's it used to turn to "cheese" on our warehouse shelves. Later polyurethanes have additives to protect them.
Baz
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BE AN INDEPENDENT THINKER, DON'T FOLLOW THE CROWD