Put the slices of rye-bread in the oven (200 degrees Celsius) for about 45 mins, until they're dried. Boil the 8 liters of water. Crumble the dried rye-bread, put it in the boiling water for about 5 mins. Let it the water, and rye-bread rest for 4 hours, covered with a tea-cloth. Crumble the yeast, 15 mins before the 4 hours are over. Mix the crumbled yeast with some sugar and the luke warm water. Let it rest for 15 mins. Filter the water-rye-bread mix in a kitchen sieve. Carefully extract all water from the rye-
For 8 hours. Sieve the solution (tea-cloth). Bottle it. Put some raisins, a bit of lemon-peel, and a fresh leaf of peppermint in every bottle, close the bottles, and keep them in a cool place. Ready when the raisins start floating. Sieve the stuff one more time in a tea- cloth. Put the Kvas in the fridge 4 hours before drinking. I got this recipe from a book called dinerparty a la perestrojka. I tried it once, it tasted terrible, but that was probably due to the fact that the rye-bread was almost burned.
This is more or less the description the book gives. Remember this is a recipe for non-brewers. It is a cookbook after all.