Chevrolet Corvette 2013 Owner's Manual page 290

10-28 Vehicle Care
If coolant is needed, add the proper
DEX-COOL coolant mixture directly
to the surge tank, but be sure the
cooling system is cool before this
is done.
1. When the cooling system,
including the coolant surge tank
pressure cap and upper radiator
hose, is no longer hot, remove
the pressure cap.
Turn the pressure cap slowly
counterclockwise about
one-quarter turn and then stop.
If a hiss is heard, wait for that to
stop. A hiss means there is still
some pressure left.
2. Keep turning the pressure cap
slowly, and remove it.
3. Fill the coolant surge tank with
the proper DEX-COOL coolant
mixture until the level inside
stabilizes at the FULL COLD
mark on the front of the
surge tank.
4. With the coolant surge tank
pressure cap off, start the
engine and let it run until the
upper radiator hose can be felt
getting hot. Any time during this
procedure, watch out for the
engine cooling fan.
By this time, the coolant level
inside the coolant surge tank
may be lower. If the level is
lower, add more of the proper
mixture to the coolant surge tank
until the level stabilizes at the
FULL COLD mark on the coolant
surge tank.