Coolant Overflow Tank Issue, Coolant Not Returning
 

Having an issue with my FD. I have a Koyo Rad and an aftermarket AST (unsure of brand, it's black and has a stant lev-r cap).
Searched a couple threads for the issue and noticed most of them are still with the stock rad or an AST with 3 nipples etc. As my issue is the AST only has 2 nipples with the bottom one that supposedly goes to the rad has been welded shut or something to that effect.
Anyways the issue, drive car, car heats up, stop driving car, car cools down, coolant is pushed to overflow tank, overflow tank fills, go to drive the next day, low coolant warning buzzer comes on, top up coolant, continue driving etc and that cycle occurs a couple of times until my overflow tank is full and I turn off the car and come back to find the coolant has overflowed and leaked out of the overflow tank.

Obviously the vacuum that's being created isn't enough to pull the coolant back into the filler neck or wherever it goes.

What should the hose routing of the AST look like if I only have 2 nipples and only have a hose going from the filler neck to ast and ast to overflow tank? (Filler neck to AST enters AST at midway point on bottle, AST to overflow tank exits at the top of the bottle right below the cap, nipple on bottom of bottle has been welded shut as there is no nipples on my rad
Should I even bother with this or should i just route my overflow tank directly to the filler neck and eliminate the AST? (prefer to keep)
Should the AST be level with, below or above the filler neck? Mine is currently sitting lower by about 4-6 inches.
Where do you have your AST mounted?

The pressure relief cap (which is mounted on the AST with a stock system) should be at the highest point of the system. Coolant expands when it gets warm, and this causes pressure in the cooling system to increase. The pressure-relief cap has a spring inside that will vent when the pressure reaches a certain amount, usually 13psi with the stock cap. If the cap is at the highest point in the system, air bubbles should travel there so it will vent air (and not coolant) when it first opens. This is good because air bubbles don't help the system transfer heat from the metal engine parts. When the engine cools down, the hose exiting the pressure relief cap will pull vacuum as it contracts (assuming the other cooling hoses aren't leaking). The length of hose between the AST cap and the overflow tank isn't easy to see, but if it's not sealing properly the vacuum from the contracting coolant won't pull fluid back into the AST from the overflow tank.



The missing nipple/hose from the bottom will prevent much fluid from flowing through the AST, so it won't do a good job of separating air from the coolant. Personally I would remove your AST and use the FC filler neck (with overflow port) , if your thermostat housing is the two-piece design that allows for the original filler neck to be replaced.

There is lots of good cooling system info on this site, here is a page specifically talking about pressure cap location.
Stewart Components

If your AST cap isnt sealing properly, coolant wont return to the system. A leaking seal could be a bad cap, or a warped ast. The Petit ast is flimsy and warps easily. If you have one of those, you should probably replace it.

Apologies for the double post. Phone was acting up when I made the first attempt and it lost what I'd wrote.

Stock Ast to Test
 

Have you tried borrowing a stock ast and mounting it properly to see if that solves the issue?

only know 3 guys local to me in kelowna with FD's and none that i know have a stock ast unfortunately.

Before you go to a lot of trouble I think I'd pressure test my cooling system and check the hose running to the overflow bottle for any kinks or obstructions. There are other reasons coolant can get stranded in your overflow.
But yes, the pictured 86-88 cap with nipple is what you want to eliminate the AST. You'll have to plug the nipple on the filler neck as well as the bottom of the radiator. And I'd ditch that 1.3 bar HKS labeled cap as well for the new OEM .9 bar cap that should come with the 'kit'.

SgtBlue gives good advise as always. That looks like the same AST I have. I've got no problems pulling coolant from the overflow tank and my AST stays full. I'd start with the free/cheap and easy stuff first. Make sure there's no coolant in your oil and no oil in your coolant. Should be obvious if those intrusions have happened. Caps and hoses would be next on my hit list. Then a long, hard look at the tanks and their respective nipples. Maybe check the nipples while you're inspecting hoses since that falls into the free category.