[shorod]

If your cooling system has only dropped one psi in 5 hours, there either isn't a leak to speak of or it will only show up when the engine is warm.

What is the system rated for? It should be on the pressure cap. I'd be surprised if it's more than 16psi. If that's the case, you really don't want to keep the pressure at 19 psi for too long or you might cause leaks that you don't already have.

-Rod