Finding Small Leaks

Did a lot of searching without a good answer - maybe you guys can help!

I'm putting together a new heating system for the house, based on a Trinity boiler with seperate loops for house heat and an indirect water heater. I've assembled the boiler, circulators, relief valves, fill valve, etc etc on a 48x48 board on my workbench for testing. Not wishing to use water yet, I pumped in air up to 25 psi though a bleeder. I was able to find and fix a couple of leaks by soap bubbles or by the faint high-pitched noise. Now I'm down to tiny leaks that make no audible noise, and blow bubbles too slowly to notice. The pressure bleeds down maybe one psi per day, so I still have leaks.

An expensive ultrasonic leak detector would work, if I could afford it. How else can I find the leaks? I see there are smoke bombs made more for sewer systems - a vendor didn't think it would work here. Would oil of wintergreen work? I'm trying to imagine how to put the stuff into the system without stinking up the whole shop!

Any other ideas?

I pump gas lines up to maybe 15lbs (depending on work area) and have no prob. findings leaks.

What I use is Polmolive dish washing liquid with a little water, mix it until I get a good sudsy solution and paint the joints, if you do this and you still have leaks start checking the body of the fittings, luck

Go to a toy store and buy a bottle of "bubble's liquid". It contains glycerin so it sticks around longer and will usually do a much better job of finding tiny leaks.