Pressure tested cooling system

[james250]

I changed my radiator with a new one, I dont think I have managed to 100% get rid of all the air yet as it took a long time for thermostat to open up because my inside heating was on MAX with A/C off. So i'm going to attempt to burp and bleed it with the funnel method again today. But I thought i would try out a pressure tester that I purchased a while back. I connected it all up and pumped the hand pump to pressurize to 15psi (i think it's supposed to be 16 psi, but i thought i'd play it safe) Actually it was on the right side of the 15 mark and then about 15minutes later i checked it, it was on the left side of the 15 mark, then i left it on there for another 35 minutes meaning a total of 50 minutes and it was still around the 15 mark on the left side of the mark. I would say that in the 50minutes it probably lost 0.8 psi and 90% of this drop happened in the first 15minutes, the remaining time it was rather steady. I have attached a picture illustration below of before and after. 

I cannot see any visible leaks on the radiator, the hoses or the water pump. Any advice? Is the pressure supposed to remain spot on the entire time? Could air in the system cause this? 

Also this was done with a cold engine with engine sat over night cooling down.