Yes, but if its not audibly coming from the exhaust, the carb, or the crankcase, could it not just be leaking right at the valve enough to cause the leak but not really have it felt or heard in the usual spots?
OK...I have a hail Mary play, if the mill is still together. If so...hook up the air, pump up the cylinder that won't hold pressure, and look in the radiator cap. Bet a dollar you'll see bubbles. :(