There is no break in period for use of drum brakes. The cables may stretch the first few times, just like any other bike. The brake cable wires should be tight and you can always use the adjusting barrels to take up slack. These brakes should stop on a dime. If they are still not working correctly, replace the existing brake cables with top quality ones that have more wire in them and better outer core. Check that the lever arm where the cable attaches to at the drum is moving easily.