One other thing that may be important to reducing air bells is to load an empty reel in first on the bottom and then place the reel with the film on top. Then you have to have a full tank. I use metal reels and tanks and have used this method for many years. No air bells at all.
Oh, with regards to mixing 1:31 use a pipette or some sort of measuring device that is more accurate than just "eye-balling" it. 1:31 is much more frugal in the long run than the way Kodak wants you to mix it.