I know this is a old thread but if this problem hasn't bin solved and your corvette is a 86-89 read this a couple of times until this makes sense to you. It does make sense. It is written by Gorden Lillebrew that I am guessing is a GM troubleshooter buy the way it is written. sorry it is long but very Good info here.

The 1986-1989 low coolant light coming on is a common problem. The green wire going into the radiator is normally grounded by coolant touching the probe wire. Usually what happens is when the thermostat opens at 195 degrees, if there is any air in the system the coolant probe becomes ungrounded. This causes the light to come on.
The radiator is actually higher then the engine; therefore, if you take the radiator cap off and the coolant doesn't run out, ( it is just Laval with cap), then you have a 2 inch air pocket across the top of the radiator. When the thermostat opens this air pocket is what makes the coolant light come on. This air pocket is caused by air being taken in to the system on cool down.
After testing in the plant we discovered that the black heater and radiator hose clamps loose torque and allow air to be sucked into the system. This is a sealed system. Any coolant expelled through the radiator cap into the overflow bottle will be sucked back into the radiator on cool down, called purging. To prevent this air being sucked back into the system, do the following.
1. Replace all black hose clamps with shiny stainless steel hose clamps.
2. Put 2 small adjustable stainless steel clamps on the hos of the overflow bottle.
3. Put 2 small adjustable stainless steel on the tip coming out if the radiator going to the overflow bottle.
4. When you fill the radiator with the engine running, in park or neutral, hold the rpms at 1500.
5. When the radiator is full, with the rpms at 1500, put the radiator cap on.
6. Let the engine go back to idle, AC off, when the coolant fan comes on at 228 degrees, shut the engine off and let it set. This will purge any air out of the system and into the overflow bottle.
7. Keep the overflow bottle about half full at all times so no air can be sucked back into the system.
8. When the engine cools down coolant will be sucked back into the engine replacing any air.
9. With a full system and no air, when you take the cap off, col lent should run out because there is no air pocket. Then you will have to purge the system again to get rid of the air pocket you just created when the coolant ran out.

I finding number 9 to be saying you will have to purge every time you open the cap
hope this my help some
Ric w.