just idle the engine, any you will bet 12.5 pSI , (the rad cap does that)
or drive around block and then idle it and just look.
you will see the leak. 12 lbs is plenty of test pressure , really.
if not ,then the you have a bad head gasket or cracked block or head.
are you running the proper ration of Antifreeze and water.
they sell a Radiator Hydrometer for $2 at most stores.
there is another test kit that attaches to the rad cap called the combustion gas tester.
if you have any leak at all, from water jacket to cylinder , it will show this fact.
looking in the overflow tank for brown or worse black soot is another clue.
the tick is to not leave the drive way, so you can see the puddle.
a compression test may help too. all cylinders must be equal. look careful.
look at all hoses , even the hidden ones, and on the right side, fire will inside and outside for the green puddles near the Cab heater radiator.