Yes, I know that you're supposed to replace the AST with an upgraded one or eliminated it completely. After all, they say you never know when it will fail. But, not wanting to spend the $$ until I had to, I just kept an eye on it.
One day the low coolant warning went off right after a cold start. I shut down and looked under the caps, the AST was virtually empty and there was no coolant visible in the block filler. But, the recovery bottle was OVERFULL and there was no sign of coolant leaking anywhere. So, I tried replacing the pressure cap on the AST, that wasn't it. For the next few days I would move coolant from the recovery bottle back into the system with a big syringe (kind of comical, no?), while I cyphered on the problem.
Then I figured it out, air was leaking INTO the cooling system during cooldown when it was supposed to be recovering coolant from the bottle (the system is under a vacuum at this time)! During one of my transfer exercises, I finally noticed a faint trace of coolant at the seam of the stock AST.
Punch line: I bought a Pettit upgraded AST and installed it in about 30 minutes last night. All is well again.:o

 

Good catch man. Way to save another FD from death by fire. I gotta do that at some point...after 1/1 probably.

 

Thats why im installing my aluminum AST Saturday.

 

I've been experiencing a simlar problem, altough I already eliminated my stock AST tank.
With every cold start up, the low coolend light will come on, and go off after the car warms up. The resevoir tank is full, and I don't see a leak anywhere.
Do you think I need to install a aluminum AST tank?

 

Make sure you're topped off at the filler neck too. You could have air in the system or a bum sensor (mine's gone crazy on me before and actually gone off BEFORE starting the car, *shrug*). If its air, just drive it around and keep topping it off, eventually the air will work its way out.