Radiator leaking slowly into oil pan.

In the process of a doing a complete rebuild of my Farmall Super A.
Before rebuild of the engine I was getting water into the oil pan. Took
head in had new valve guides, valves ground, resurfaced and magna
fluxed.
New sleeves in engine and turned crank, new pistons and so forth.
After sleeves installed and without head and no radiator I plugged
lower radiator drain and filled engine with water to look for leaks.
This was about a 30 minute look. There was no obvious leaking.
Put engine together with new head gasket. Installed radiator and
filled with antifreeze. Let it sit overnight and in the morning the
radiator was down about 1/2 inch in the neck of the radiator. Thought
maybe there had been a air burp come up. Note tractor has not been
turned over or run yet. Refilled radiator and after a week the radiator
is down another 1/2 inch. Pull the pan and there is antifreeze in the
oil pan
So looking inside the bottom half of the engine I cannot see anything
obvious partly because leak is so slow.
Yes I did clean groove where sleeve O ring sits
Has anyone ever pressurized radiator with 10-15 pounds of air
pressure to look for leak?
I assume the only place for leaks are around bottom of sleeves, thru
head gasket or a crack in block somewhere.
Is there and hope when the engine heats up and things start moving
that it might seal itself?
Should I consider putting some radiator seal in.
This tractor has no water pump just thermo syphoning

& to 8 psi is fine (not more) a radiator pressure test pump is the way to go. More may burst the radiator tank or core. Put a clean cardboard under the open bottom end to see where the drip hits. Jim

Thank you for the suggestions. That?s wonderful

When I did my H it had been froze with a cracked head after diff. head & new gasket & diff. radiator I used pressure tester with pan off & 8 pounds for about a week. no water and that was in 2010 has been fine since. Geno

When I worked for an Allis Chalmers dealer we overhauled about three WD's or WD45's a week. A tube of Aluma-seal went into the radiator of all of them. Shop foreman was considered by most farmers to be the very best, but I never had much appreciation for him. he was very opinionated and made a diet of backsides

If you can't find it with a little pressure I would drain all the antifreeze and refill with straight water and some stop-leak and run it. That small a leak should seal right up. Many new vehicles come off the assembly line with some sort of stop leak in them, it works!