Not Ranked
I can be wrong but I believe the cause for this is the stainless top pipe, electrochemically reacting with the aluminium block.
I can only think the reason for this going away is due to the Manganese (in the stainless) being removed (from the surface) and thus causing the reaction to stop.
Next time you have the top hose off have a look inside, I'd be curious to know if it looks different (maybe black or tan tinge)
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