There have been a lot of threads on this "problem" and many have had slight leaks when changing from original rads to new aluminium rads.
I had the same on my Zetec and to this day I get a very slight weeping especially on very cold days when the aluminium seems to contract a bit.
Although mine was annoying, it seems to have settled down now and the water loss is negligible.
I changed to silicone hoses at the same time.
