If you look down into the battery there should be a tube extending down into the battery under each cap. There shoul be a notch in each of these tubes. The water level should be above the lower end of the tube but below the top of the notch.
When you charge a battery Hydrogen gas is generated and the water/acid mixture is heated up. If the fluid level is too high (above the top of the notch in the tube) the hydrogen can't get out of the battery and collects up under the top of the battery case. Eventually the gas builds up enough pressure and pushes the fluid level down and up the fill tubes. This is usually why acid runs out the top of the breather caps. Over charging a battery with a normal level can do this as well.
1969 Indy Pace Car
350/300HP RPO Z11 My 69 Pace Car