My 1990 3.0L has a little over 10k+ miles on a full engine rebuild. It runs great with no apparent problems. However, I'm having an issue with coolant disappearing. It hasn't done this since the rebuild, but over the last six months or so it's become an issue.

What I've observed is that I top off the overflow reservoir and then drive it for a week or so (probably ~120 miles/week); then when I check it again, without fail the overflow reservoir will be empty. But when I open the radiator, it is never below full, even after running the car for several weeks or multiple hundreds of miles. For example, I took it to Big Bend and back a couple of weekends ago; 500 miles one way with plenty of 80+ mph driving en-route. I only topped the overflow off once while I was in the park.

I've never seen the radiator anything short of full; in fact frequently when I go to open the cap it "pops" off as if it's still under pressure. And sometimes it's (over) full: i.e. all the way to the brim. The cap is brand new (AutoZone) and I even replaced it with another new one to make sure it wasn't the cause of the problem

I've put a pressure tester on the system and let it sit for several hours. It loses pressure at the rate of a couple of lbs/hour. But I'm not seeing it drip anywhere (skid plates were off). I know all of the places where it's likely to drip (heater hoses, TB hoses) and even getting my hand around there I don't feel it getting wet anywhere. It did have some leaks around some of the hose clamps, but I've closed all of those that I could find.

The only ideas I can come up with are that it's leaking in a place where I can't see or feel. I guess that would mean internal (e.g. head gasket) or in the valley under the intake and between the heads?

I also think that it's weird that it always seems to slurp every bit of coolant out of the overflow but then never seems to go down in the radiator. Does that make sense in anyone's experience? Any other suggestions?

thanks,
steve