Pressure tester

A common problem on GM engines is a leaking intake manifold where the coolant passes through passages in it. Those leaks usually occur in places where you can't really see it. In this case it might be leaking fast enough to prevent your tester from building pressure.

An alternative is to add a small bottle of dark purple dye to the coolant, then check a day later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Auto parts stores will have the correct dye for the fluid being tested, and those that rent or borrow tools should have a black light. Typically you pay for the tool, then you get a full refund when you take it back. If you decide to keep a tool, you still take it back, then they order you a brand new one.

If you see where it's leaking from, the diagnosis is already done. If you only see where it drips from after it runs down there, that is the exact purpose of using dye. A pressure tester won't help because that just gives you more of what you already have, meaning leakage. Once the liquid evaporates or boils off, the dye is left behind. While looking at it with the black light, it's that bright yellow stain that you follow back to the source to see where it originated from. Remember that liquid always runs down, so look for the highest place you find the dye.

If I can jump in here. this guide shows how to use a pressure tester and to find a coolant leak:

https://www.2carpros.com/articles/car-is-leaking-coolant

Please run down this guide and report back.