I think there is a drain hole in the heater box, for the evaporator when it defrosts...so you might not see a leak onto your floor. I would maybe try eliminating the heater core by either pinching the lines to it or plugging them, then pressure test it again. If it holds pressure with the core eliminated, then you know your leak is in the core. I usually use some cut off broom handles about 5" long and then take 2 of them and drill and bolt them together like a clamp. They are great when you need to disconnect a line and dont want to loose your coolant, and would allow you to clamp the heater core hoses without damaging them.
