What you're describing is one of the things a leak down tester does.
Google leak down tester ... there are instructions for making your own
and their uses.

Mike

Bob Mac wrote:
> 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.
>