Norm I was speaking professional way testing that eliminates those that seep that don't under such a test as you describe. I use the same method and after several comebacks I found it was wise to get the equipment and be 100% sure there is no seepage. At first I had a few false positives but it was that I wasn't using clamps. It has been money well spent.
When I did the gravity test you are describing it might be 24 hrs before a problem show up. It one the reason I always left the carburetor loose on the engine overnight with tissue underneath the carburetor leaks would be easily seen even when things were dry the next morning. The reason is that a fluid doesn't leak as easily as air due to surface tension. It will tend to seal a hairline problem until it dries past that point.
This tester I am using now finds even hairline scratches that causes seepage which Kohler Commands and Courages with Walbro carburetors that tend to seep when the needle is worn. This is a bad enough that I keep the kits in stock at all times for these Kohlers.
