When I bought Gerry's FD [S/M Six rotor] I'm positifve he said he had bypassed it, but I can't figure out how he did it - he told me a decade ago and even then it was information overload.

 

You can delete the FD's AST by using a 2nd gen filler neck and putting the pressure cap and line to the overflow there.

It was a thing to do 10-15 years ago because the stock plastic units tend to wear out and crack. No bueno. A simple fix is just to use a metal replacement.

The perk of having an AST is that it tends to collect and burp air bubbles. You can get away with removing it, you just need to be extra careful about burping your coolant.