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Update. The heads are new and held pressure. Then we aired up the block through the rear ports that feed the heads. It doesn't hold. No apparent external leaks. The rivets were removed as the gaskets were disassembled an sprayed. The motors coming out this afternoon. Pressure added to the cooling system forces oil from the oil feeds. There also appears to be some coolant in the oil.
Doug

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There is no way that the motor will hold air pressure through the oil system, bearing clearances, lifter clearances, blowing the oil back through the pump and filter into the pan and so on That might be a way to find the leak though, look and feel for the escaping air