One of the problems with our homemade tanker was getting air into the tank when dumping. We had to climb onto the top of the tank and open one of the lids. This sucked when it was rainy, cold, or icy. We decided to make some automatic vents.
This what we came up with. The chief found some plastic collars that matched well and didn't leak. We removed two lids and put the vents there. When the water goes down the vents lower and let in air. It works great. It also helps in filling the tank as you can tell when the tank is almost full by watching the sticks rise. Simple and effective.