I found both dark and light rye bread at the store but it was the factory-made kind. I definitely made mistakes with both batches including adding way too much bread to the light kvass.the bottles need a day to carbonate then we shall see how well they turned out. Not that I have any reference for the flavor.
I used new bread each time, it was both sweet and sour with the the light rye being the sweeter of the two. I tried again with homemade rye bread and neither attempts were pleasant enough for my palate for me to make this again.