I think you are using the primer caps with the chamfer on the seat so the needle will seal ,if not I can see the problem ,
Might be easier trying a pressure tester ,I was thinking of making one and using a Doctors blood pressure tester hand pump ,the pump is only a few dollars online and just hook it to a pressure gauge. That or just use a bike pump connected with a gauge.
The gravity feed idea should work it's just the time that it takes to wait.
I haven't had this problem ,if the cap leaks I replace the needle ,but I never buy new caps always just take a second hand cap off a mower ,I know there are a lot of problems with low quality aftermarket caps, the only way is to buy them from the same supplier once you know they are ok.
The pressure gauge would be a quicker test and is easier when testing the same needle in different caps.
I think you saw this video Norm showing the different types of caps that look the same on the outside.
I'm sure once you can get the caps and needles to the right specs you shouldn't have a problem Norm but that may take some work as you say polishing the seat with Autosole may help but with a pressure tester you can see the result straight away.
