Having the valve cover off wouldn't allow you to see any cracks unless it is tremendously catastrophic. The cracks are on the combustion chamber side. I am not real sure about the pressure test as a check, either. Mine would hold pressure in the system for days, that is why the upper radiator hose is hard when cold, it is holding pressure. I think the truly best test is checking for combustion gases in the cooling system, but the circumstantial evidence is usually enough to prove the problem.

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Keep everything as simple as possible-but no simpler--Albert Einstein