The more solution you put in the bottles the less likely they are to colapse because the weight of the fluid holds them in place. However the more solution you put in the bottle the more likely it is to get sucked into the tank when the bottle does colapse. A definate catch 22. The sad part is that a check valve can't prevent this because it sees pressure on one side or vacuum on the other as the same thing a difference in pressure going in the same direction. The best solution to this problem I have found is to use more rigid bottles, like the thicker plastic bottles that Gatorade comes in. It takes a lot more of a pressure difference to colapse a Gatorade bottle than it does to colapse a 2 liter soda bottle. Definately replace the bottle that colapsed on you, because once the bottle has been colapsed or kinked it will colapse under a lower pressure difference than a newer unkinked bottle.