I had exactly the problem Dennis Haynes described with air being sucked
into the system after doing my conversion.  There were no visible leaks
that I could find, although when hot, I could smell coolant...just couldn't
find the leak.

This persisted for quite a while before I finally got fed up and made a
simple pressurizing tool.

I used a standard air nipple clamped to a rubber hose.  This hose was
clamped to the overflow port on the expansion tank cap (goes to the
overflow tank).  15 psi of air, and lo and behold...leaks from the large
pipe clamps going forward to the rad...and gushers at 15 psi.

Don't mess around, follow Dennis H's advise.  Pressure test, and if that's
fine, do a leak down.  My air leaks are gone...