I cannot remember and am too lazy to find the thread about preventing this from happening. Mud and junk enters this hole easily as it points directly to the front wheel. The mud and junk grinds away at the seals untill they fail. With some small tubing you can install a down tube which will allow leaking if the seals fail ( no contamination of the oil with coolant ) and stop the junk entering.
Re changing the seal, daleEO has posted some invaluable information regarding this.
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