sounds like your water pump has a bad seal in it,it has something called a seep hole that equalizes air pressure if the pump seal are good,if not it it allows it too leak,now just because it does not get hot does not mean your water pump is not bad,soon the bearing in the pump will give way and it will start running hot,the pressure test will not detect that leak,what you do is start your car leave it running for a bit until your thermostat opens up,that is when your pump starts working,now look at the back of the pump you should see a constant trickle,if so it has a bad seal