Disappearing AND REAPPEARING coolant

[Guam180]

Ok this is a little odd:

Over the weekend, my car sat. I didn't even start her up or any thing. Yesterday, I check her under the hood and notice the coolant level is low in the overflow tank. I fill it up, and ensure that the radiator itself is fine, which it was. Today, I check it again, and now it is is OVER THE FILL LINE that I fill it up to yesterday!

This has actually been going on for some time now but I haven't had the time to do anything, so I've been dealing with it.

1991 240sx fastback
Swapped in a '96 KA about 3 months ago
replaced radiator with a koyo oem replacement
new hose and clamps from radiator to overflow tank (coolant used to just disappear and had to refill daily. Reappearing issue happened after replacing this)

It can't be a leak, since the fluid can't jump back into the reservior. But where the hell was all that coolant over the weekend? At both times, I checked when the engine was cold, resting at least overnight.

Any ideas anyone?

[seang]

My guess is that hot coolant is pushing past the radiator cap into the overflow tank, and getting pulled back into the system when everything cools down. Are you still using the original radiator cap?

[Guam180]

The coolant is overfilled in the overflow tank, when the engine is cold, AFTER LEAVING IT OVERNIGHT! I'm sorry, but I don't think you read my post right. I'm familiar with how the overflow tank is suppose to function. In this case the opposite is happening.

[canyouSAY240]

is your car over heating or anything like that.? maybe its getting cought in the block?

[biggie]

Bad rad cap.