There is NO problem with driving like this.  I had the sensor leak and when
I tightened it stripped out.  I was over 200 miles from home when this
happened.  It leaked some coolant down to the level of the sensor hole.  I
checked it several times and after that I did not worry about it.  Air
cannot get into the system unless it DROPS below the MIN line on the
pressure tank.  This doesn't mean to not get it fixed, BUT it is OK to drive
it until it is fixed.  If the tank is cracked see where the crack is
located.

Robert