2 Pack Air Release Valve 3/4 Inch Air and Vacuum Release Male Thread Air Relief Air Bleeder Valve Vent Irrigation Water Saving System for Agricultural Drip Systems

It took me a few years to figure out how to vent my horizontal barrels to get them 100% full instead of having the last 5 gallons be unusable air space at the upper part. You need to make a vent at the end of the barrel just as if you planned to put a tap, drain, or spigot there with it standing on end.For the lower level of stacked barrels; attach a length of hose that has to go higher than the level of the fill tube. That vents the barrel so that it fills completely. Getting the vent hoses set up properly, and making them bug proof takes some trial and error. And, makes cleaning the barrels a chore.I'm in the process of re-doing my 4 barrel rack (2 tiers of 2 barrels) so I will need to leave enough space between levels to fit these valves under the upper tier.I use the rubber inserts that allow you to do all the work from outside the barrels. That also eliminates the need to use the nylon tape that came with these valves. The rubber makes a nice seal; but screwing these valves in took a couple of tries until I added a a couple of drops of vegetable oil to the threads. They went in perfectly after that.These valves work in the same way a wet dry shop-vac does to protect the motor. There is a float ball inside that lets air flow out until the water pushes it all the way up; and closes the valve. And, when the water goes down; it prevents a vacuum from forming in the barrel (Trust me, you don't want that to happen) and that allows the water to flow normally.Also, when a barrel isn't vented properly, it will compress the trapped air inside that cause problems with pressure leaks at all your seals. These valves takes care of that too. So, it would have been worth it for me to have gotten some from the start.