Originally Posted by alabbasi

It sounds to me like you have either a head gasket or intake manifold gasket leak. It may be slight.

The pressure tester would be one way of testing it as it will either force coolant out of it's leak point if it's external or into the block, thus causing a pressure drop.

Another means to test for HG leaks is to the chemical block testers like the one below:

Cylinder Head Gasket Block Tester Kit | eBay