breaking a vacume

[jslaney]

So I will be installing my plumbing tonight and I want to cofirm that the only way to prevent your return line from vacuming half the water out of your tank should your pump fail is to drill a hole at the highest point on your return line. How big should that hole be. I am assuming I want to drill it in a direction such that water does not squirt out the top.

Clear as mud?

Thanks,

Jon.