Nope, not your doing at all. You got some good options to pursue from this thread.

 

Well you did, Jowens. You stirred crap up. You're a sht stirrer, Jowens. I tell ya. Some people...

No, I'm just kidding. Heh heh.

I always find the best discussions are the ones that ultimately tend to expand into more intricate factors when folks are supporting their theories. I learn more that way, just by reading through them. I say theories for lack of a better word.

Anyway. All I'll add here is that my 1990/L98 holds exactly 17.8 quarts of coolant after the knock sensors are removed to completely drain and flush the coolant system. It has a stock radiator but I also removed the oil cooler completely and did a throttle body bypass. So now it holds around 17.25 quarts of coolant when filled from a completely empty system.

When I fill it I remove the thermostat and fill it directly into the block. Then I finish it off by filling the radiator the rest of the way through the return hose. Then I top it off at the reservoir. Then I fill the overflow tank to the cold line. Then top it off again at the reservoir after a good drive up to temps.

As was mentioned, the radiator cap is a cheap replacement, and often the problem. Mine uses a 17lb closed system cap.

Also a good idea to check the neck of your coolant fill reservoir for any cracks. Also a good idea to check the coolant fill reservior itself for leaks. You probably won't be able to actually see a leak there since it happens when the coolant comes up to temperature.

A pressure check is usually where I'd personally start investigating.