In desperation - plus all of the above - get some plasticene... build a little dam around the top fittings... fill with water/detergent, or better still, get some bubble solution for kid's bubble toys... You could try strong or 'neat' detergent/bubble stuff, stickier and may stay in place long enough to see a bubble forming.
Trust nothing, even a new fitting can leak (guess how I know)

An ultrasonic leak detector is problematical, the velocity of escaping air not normally high enough to generate sufficient 'noise'. Worked on a couple of my leaks, but mis ed one which I found with the plasticene.

Look on the bright side. You will go through hell and high water to finally nail the faults..and when it finally happens...the feeling of Accomplishment and Omnipotence is priceless !!!