I have just bought a new Dishwasher, but cannot get spare racks.Can I use it for both Milchik and Fleishik( Seperatly) if I put it through an empty cycle between washes ?

It’s highly recommended not to use the dishwasher for both milk and meat without changing the racks and best if you use it for only milk or meat. If you do use it for both it will not make the dishes non-kosher.