Thomas,
Which container was low? The one behind the license plate (aka overflow
tank)  or the one with the pressure cap (aka coolant reservoir) ? The
reservoir should always be full to the top with NO air.  This one contains
the level sensor (at least in my 86, may be different in an older diesel).
The level  in the overflow tank  can and will vary.

    Adding two cups to the overflow will eventually be drawn in to the
system so keep an eye on it.

Ken Lewis
http://neksiwel.20m.com/