[m6-lt1]

One thing I'll point out is you may have never had a problem when you had the original tank but the new tank is defective and now you have a problem. Here's why:

I thought my lower tank was losing coolant at a fast rate and was concerned. Dealership told me they filled it but I have a feeling they did not. Anyway bought 50/50 dexcool and filled it to the exact line myself. I hadn't driven the car for a few days when I filled it.

Now after a drive I notice the lower tank is filled PASSED the line (about 2/3) which tells me I filled it to much/didn't account for coolant in other places. During my last track day I noticed the lower tank was filled up to 3/4 and got concerned. After the next session it had dropped back down to 2/3.

If the car hasn't run for a while the coolant in the lower reservoir is exactly at the line just like when I originally filled it even though it was at 2/3 when I shut off.

I've seen a lot of posts here about the coolant in that tank being low and I think it's due to the coolant slowly going back into the motor or radiator after the car has been off for awhile. When you replace the tank again factor in what I just mentioned if it is lower after not driving it for awhile. I jsut bought a syringe wo suck some of the coolant out because I do think mine is overfilled a bit even though it is exactly at the line after the car hasn't run for a few days.