I just replaced the thermostat and used my pressure tester to make sure it wasn't leaking around the flange. Then I discovered it would not hold 15 Lbs. of pressure, but I don't see it leaking anywhere? Ideas?

Then its leaking internally, like the intake manifold gaskets. Keep an eye on your oil level and coolant level.

Exactly what I thought....those 4.3 where known for intake manifold gasket problems....but it thru me off because the level of coolant in the radiator (and tank) was not down any....may be just a new problem..... I'll tell him to keep an eye on it.