You might try this. The compression test may look good because if the
gasket is leaking into the coolant that mean the compression is backed
up by the coolant, hence no leak. We used this to replace valve springs
without taking the head off but i think it will work in this application
as well. Make an adaptor for your air chuck to fit in the sparkplug
hole. Use an old plug and break the porcelain off and braze on a male
end of the air chuck. Thread this into each spark plug hole one at a
time. make sure the valves are closed in that particular cylinder then
put the air on the adaptor. This will pump air into the cylinder and
should force air thru any gasket that is bad. you should see air bubbles
coming back into the coolant tank after a short while. Don't laugh try
it.