Radiator Low, Overflow Tank Full

[89AccordLX89]

1989 Honda Accord LX
142,xxx miles

Been having some trouble with my car's cooling system. It has not been over-heating whatsoever so I am a little stumped
When I check the radiator after any distance drive it is always low and the overflow tank is always completely full
I am not leaking any coolant on the ground that I can see, bleed the cooling system of any air, and the oil is always clean, not milky at all
Just replaced the thermostat today thinking that had something to do with it but am still having the same symptoms

If anyone could help I would really appreciate it, just want the old girl to be running at 100%!!!

[Dr_Snooz]

What does your radiator cap look like? It might be time to replace it.

[89AccordLX89]

Completely forgot to mention that I did replace it with a new one when I put in a new thermostat
I'm pretty stumped

[w261w261]

Did you first notice the problem and then do the things like the thermostat and cap, and it didn't help? Or did you do some work and then notice the level problem? When you first noticed it, had you started with a full radiator and a half-filled overflow bottle? Then after you drove and let it sit, was that when the bottle filled up and the radiator level dropped? Or did you start out with a full overflow bottle and a full radiator?

If you started with the bottle less than half-filled (that's what you should do, only fill to the "cold" level), and then it filled up with the radiator overflow after stopping (on a fully warmed up engine), then the system is functioning for the overflow, but might not be on the siphon coming back. Check on whether the tube that goes down inside the bottle goes all the way in. If it has been pulled partially out, then the coolant would blow into the bottle after boiling in the block and pushing open the cap, but would not be able to be siphoned back into the radiator as things cool down. In other words, it would only work in one direction.