1940 D8 water in pony motor oil

I have had pony motor issues with water getting into the oil. Last fall I pulled the head and had it magnafluxed, and replaced the head gasket. I did a dye flux test of the top of the block and no cracks. took the inspection plate off in front of the valves and its dry in there. Pony ran great and I was able to get the main engine started but had to shut it down when the fan belt broke. 10 months later I finally have some time to spend on it. Got the fan belt replaced. Checked the oil in the pony and its full of water again. My question is where would water be coming into the pony motor oil if the head gasket is good, the head has no cracks, and it doesn't appear the block is cracked?  Is it possible the water in the oil pan was left over from before I replaced the head gasket in a low spot of the pan, and I just didn't evacuate all of it by removing the drain plug? the reason I wonder that is I originally pulled the oil drain plug in May and drained the watery oil, so I thought I was good, and put the plug back in. I did not replace any oil at that point. It sat since then with no oil added. It wasn't until this last weekend that I pulled the oil stick out to look at it and saw it measured water/oil on it still.  So I took the plug back out and blew air down the oil stick hole and a couple cups of water came out. I had checked the bottom of the oil pan with a stiff wire to make sure there was nothing blocking the drain before I blew air through so that was a surprise. I am just wondering before I put new oil in if its possible there was a low area in the pan that didn't drain out to begin with, or if I need to keep tracking down the source of the water. Sorry this is so lengthy but thought I would add as much information as possible. Thanks in advance for any suggestions. 

The block could be crack inside

Thanks for the response gemdozer. I have to admit my theory of water leftover in a low spot was wishful thinking. I am not looking forward to pulling the pony motor.

is it possible to pressure test the cooling system? that might help diagnose a cracked block.

you can also get test kits to check for combustion products in the coolant.
that would only show a crack from cyl to water jacket though.

thanks for the suggestions. I am not certain how to pressure test the cooling system since the main and the pony share the system. I am going to remove the lower inspection plate from the pony motor this weekend and see if I can see anything from there. Not sure how successful it will be. Probably going to end up pulling the pony after one more start of the main this year.

Possibility?
Dad had a 9GD7 that had water in the oil on the Starting Engine.
Two petcocks on the bottom dippers - so drained it before starting and kept oil full. Not ideal situation.
That particular case, the water jacket was cracked behind the valves. Difficult to see without having the Starting engine off and dissasembled.

Chad Enyeart (was-is) the owner of that particular tractor, you might contact him for information on how he found and repaired the problem.
Antique Caterpillar Parts - Laramie, Wy last I had heard.

Hope this helps.
CTS