The caps are the only way for pressure to relieve or enter the tanks. The outlets at the top of each tank (shown in the following two photos) are connected to the caps.
Cut out the two flaps (marked with orange X's)that are next to each other. This will allow the tank to vent equally in both directions. Otherwise the exit pressure has to pass through the squiggly labyrinth shown in InnerCap.jpg
Photo by Goss

The caps are the only way for pressure to relieve or enter the tanks. The outlets at the top of each tank (shown in the following two photos) are connected to the caps.
Cut out the two flaps (marked with orange X's)that are next to each other. This will allow the tank to vent equally in both directions. Otherwise the exit pressure has to pass through the squiggly labyrinth shown in InnerCap.jpg
Photo by Goss