What I meant by "leak" is that, when the fabric gets wet on the outside, water soaks through the fabric. It will either soak through at stitching or where there is a worn spot in the fabric's coating, which is on the inside. Anything in the pack that touches the fabric will be touching wetness, and ends up getting wet too. If you are carrying something like clothes, they will start to get wet where they touch the pack fabric, and eventually will get soaked.
Some backpacks have waterproof covers which can be stretched over the pack for rainstorms. These are too weak for going through brush.
What I do is use a dry bag (the kind used for kayaking) inside my pack. I put things that must be kept dry (toilet paper, clothes, etc.) in the dry bag. You can use a plastic garbage sack too.
In my experience there is nothing that you can do to a pack to make it waterproof. Coated fabric works a little bit toward that end, but only when it is new. Even uncoated fabric isn't that bad, in all honesty.
Scott
P.S. The main point of the backpack covers seems to be to reduce weight. You are carrying a loaded pack and a wet cover, not a wet pack, which is much heavier.