Here's when you need an AST!
 

Yes, I know that you're supposed to replace the AST with an upgraded one or eliminate 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

Hey Cliff, I havent swapped out my AST yet, but you mentioned vacuum during heatsoak/cooldown. Mine gurgles LOUDLY like a coffee pot after shutoff and seems to build-up MORE pressure, and you can feel and SEE the coolant thumping thru the thermohousing-to-AST hose. Had it refilled and burped, and it still gurgles noisily, like it's going to explode. Any ideas?

It's normal. After shutdown the car heatsoaks and the coolant in the AST and thermostat housing will boil; that's the gurgling sound. As soon as you start it up and it circulates again, it will stop right away. No need to worry unless you are losing coolant somehow.

Hey Cliff....i thought you where selling your FD? Just by installing your old pettit ecu, i dropped .2 in the 1/4. Later, CJ

Thanks for the advice Damon. I thought it was air in the resulting vacuum, even with full/burped coolant. 30 minutes of it sounds like it might explode the AST(ordered a Pettir tonight).