So far so bad.
First try was using only air, then coolant. When pressurized coolant, pressure drop was slower, but nontheless should be nonexistent. Starting point at 180 Kpa or 1.8 bar, as per cap blow off value. It held even 1.9 pressure.
https://www.youtube.com/watch?v=cZ-ImSU6yyQ
Next job is submerging whole engine, using air only in cooling system and observing bubbles.