John's explanation sounds good, but you should also realize that running the
engine for a few minutes will not move coolant from the overflow tank into the
expansion tank, a complete warming -cooling cycle is needed.
  If there is too much air in the expansion tank, it will not be driven out
until the coolant expands as the engine reaches full operating temp.  Then when
the engine cools down, the contraction of the coolant in the system will draw
fresh coolant from the overflow tank, thru the valve in the pressure cap and
into the expansion tank.
  If you don't want to wait, you can disconnect the hose from the pressure cap,
remove the cap, and top it up yourself.  Be careful about opening the pressure
cap when the engine is hot.
  Larry Alofs
    '91  GL   "Vinnie"

dianne gigler wrote:  I started the engine

> and let it run for several minutes to see if the level in the reserve tank
> went down.  It did not, so I closed things up and drove away.
>
> About three minutes later, the light came on again and the needle was below
> the half-way mark.  I checked the reserve tank level and it was still at
> the "max" level.  Against my better judgement, I decided to proceed to my
> destination which was about seven miles away.  The light remained on the
> entire time, but once again the needle did not go beyond the half-way mark.
> I made arrangements to borrow car and garaged the vanagon at my destination
> after once again checking the coolant level which had not change.