I'm recommissioning a 1973 Mercury 75 and ran into a snag. The flush port below the cowling doesn't seem to go anywhere! I've tried to run water, and even compressed air through it and got nothing.. The rest of the cooling system seems fine. I have to lower off. The impeller was in good condition. No noticeable salt build up anywhere. There is full flow out the pee hole from the top of the head. Last year the motor ran fine without overheating at all. I'm confused... What could be blocking the passage to the flush port that can withstand water pressure all night, and then 100 psi air????