The first thought that comes to mind is that the coolant is getting pushed out the reservoir overflow tube when the engine warms up because of a bad radiator cap. A pressure test won’t show this, because one has to remove the radiator cap to do the test! You can test a radiattor cap with a thermometer, a hotplate, and a Pyrex vessel. Simply hang the cap into the water, turn the hotplate on, and see what temperature the T-stat opens at.