Thanks for posting Julian. When a truck is parked on level ground the water entering the rails does indeed travel to the front drains. However, as the vehicle is driven it is constantly going up & down inclines and around corners. This causes the water in the rails to alternate between running to the front & running to the rear. If no rear drains are present the rails simply overflow into the bed. When rear drains & factory bed extender are present, the water runs out the drain tubes & into the bed also. When a bed extender is present, there is no way to accomplish "drain tubes just tuck behind the 2 attachment points where the tailgate meets the floor of the bed" as suggested. This is becuase the gap between the tail gate & floor is so small that the tubes are pinched shut when the gate is closed. So either way water is dumped into the bed. The water that is dumped into the bed is spread all over the bed as the truck is driven resulting in wet cargo stowed under the cover.

One solution that looks like it would provide the needed rear drains & be compatible with the factory bed extender is to provide a 90 degree elbow at the rear drains. If the next size smaller tube (1/2" ID vs 5/8" ID) is used, it could be routed into the smaller existing holes on the side of the bed channel located below the rails. This would allow water to be routed outside of the bed and eliminate the the interference between the drain tubes & the factory bed extender. This would complete the design & make a truly great cover.

 

Obviously driving conditions, rain conditions, etc, could lead to what the above post is saying.

In my experience through rain, car washes, etc, I have had VERY LITTLE moisture in the bed. None in the front, ever. I've had what I would say is a very small amount of water in the back after ridiculously heavy rain or high-pressure car wash. I've never had enough water that would actually pool and move around in the bed as you drive. More like moisture, water droplets, if that makes any sense.

All in all for something so low profile, it does an amazing job at keeping the bed dry - in my humble opinion.

 

Thanks for all the replies and suggestions!

I ended up ordering the Ford OEM cover from the dealer yesterday. The dealer gave me a decent discount and I was able to get it for only $25 more than my local aftermarket store was selling the regular HD. The $25 seemed well worth it for the 3-year warranty and slightly more "refined" components (according to Bak).

I'm pretty sure I'll end up routing the drain tubes into the stake pockets for a nice clean look. I spoke to Line X and they just said to drill my holes slightly larger than I need because their liner will fill them in a little bit.