when you have tiny leaks there are some much simpler, easier and more effective methods that you can use.
My preferred method is to drain the system and to pressurise it with air to about 1 or 2 bar. [When building a new system, it is tested this way before being filled with water; much easier to fix leaks on a dry system.]
If it holds pressure for a day, there is no appreciable leak.
If it does not hold pressure then there is a leak of some kind.
This test is very sensitive; a leak that wees out many litres of air may only leak water at a miniscule rate, because water is so much more viscous than air, and surface tension reduces the (low pressure) leak rate out of tiny holes too.
To find the air leak, just go over suspect joints with a brush (or trigger spray) and soapy water. Any joint that you can easily repair will be accessible in this way.
Obviously you can find leaks in engine cooling systems this way too, but in a dry system the water pump gland may leak all the time, and you may need to hook the system up to a regulator so that it holds a constant pressure whilst you are looking for leaks.
BTW to pressurise a domestic heating system, I typically use a blanking cap with a bicycle valve (Presta of course...
) set into it.
cheers