Coolant system issue multiple caps S4 w/o overflow
 

I have an interesting cooling system (see picture) and I have found a few things out.

1.My radiator system acts as two separate circuits when the thermostat is closed. Engine side of the thermostat and Radiator side of the thermostat. The lower radiator hose does connect the two but it does not equalize the circuits fast enough to prevent air from getting into the system. My system keeps the engine side full at a higher level while the radiator side collects air. If I open the cap on the each side individual the water level stays the same, is I open both the water levels out.

2.You can not use a pressure cap on the radiator side of the circuit unless you have an overflow/siphon or another pressure cap overflow/siphon on the radiator side. My radiator cools much faster than my engine, creating a large amount of vacuum on the radiator side only . This causes the pressure cap to leak atmosphere into the radiator circuit. I've glued my pressure cap shut on the radiator side and it helped but did not fix the issue. I need to pull coolant in.

I’ve tested all of my components (caps and lines) and systems for leaks and there are no noticeable issues at normal pressures. (I’m sure there are some leaks) My system only purges from the engine side, which I believe my biggest issue. Before my radiator neck on my Koyo failed I had a purge on the radiator side and I did not have this issue.

So my plan is to first to get a new larger radiator, then to add an overflow/siphon to my radiator side circuit. I have not decided whether to weld a bung on my S4 thermostat neck, buy the plastic neck with the overflow/siphon, or to buy a radiator with an integral cap.

I will also drill another hole in my thermostat to help equalize the pressures but do not think this alone will fix the issue.

Let me know if I’m on the right track.
Thanks

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