Pressure testing coolant system

My daughters stock 93 lx 5.0l auto car has always used antifreeze since we bought it 6 years ago. It was always a small amount so I never worried about it but lately it has increased considerably. I picked up a Motorad TM300 tester today and checked the cap, it tested good to 16 psi like it should. I then tested the coolant system and by the time I got to 14 psi it was falling off as fast as I could pump. It would drop down to 8 psi in a 4-5 seconds then at a much slower rate. There has never been any puddles or leaks I can find. I also started the car when it was at 8 psi to see if it would pressure up. My logic was if it was a head gasket I should see some pressure spikes but the needle was steady.

I was going to pull the plugs out tonight and see if I can see a wet plug or any signs of water. After that I have no idea what to do besides start to pull the intake off looking for a leak.

Any suggestions?

If the head gasket is leaking internally, you can usually tell by checking the tailpipes. If you wipe your finger inside the tip and it comes out clean instead of sooty, you'll know it's a blown head gasket aand should be able to tell which side is bad.
Gary