Unhooked hose from degas bottle to apply device to check head gaskets. Hissing sound came out due to pressure in system. If I head a blown head gasket... would system still hold pressure 4 to 5 days later?

 

I have has two different trucks and both would hold pressure for days.

The first truck was all stock and fresh head gaskets.
The second one I have questioned. It used coolant and
it still will hiss when you remove the cap. It's also deleted and studded.

 

Different engine, but I had an LLY Duramax that was holding pressure, turns out it was a blown head gasket.

I’ve never checked this on my truck now. I supposed I should