Coolant Mystery

I have a coolant mystery that's even stumped my mechanic.

2006 4.0, 199,000 miles. On long, hot drives at freeway speeds the coolant is backing up into the overflow reservoir but not siphoning back in when the engine cools down again.

Here're the pertinents:

-Thermostat, water pump, hoses, radiator, cap are all less than 2 years old
-No visible leaks
-New radiator cap didn't help
-No fluid loss (unless I let the overflow tank spill over) The exact amount of fluid that's backed up into the reservoir is what's needed to fill the radiator again.
-No obvious head gasket symptoms (no coolant/oil mixing, no smoke, no leaks, no fluid missing, no bubbles in the tank when engine runs)
-No overheating

All I can think is there's some sort of pinhole leak that is enough to break the vacuum when the engine cools but not big enough to leak fluid.

Any ideas? Thanks!

 

Has anyone done a pressure test ?

 

Have you tried replacing the tube that goes from the radiator cap to the reservoir?
Or perhaps the tube if not quite tight enough on the attachment at the radiator?

If the tube is clear, you should see coolant in the tube. If not, then you have a pinhole or something that is letting air in.

 

Have you replaced the radiator cap ?

Check to see if the hose under the reservoir cap is there?