Bright Star Air Mass II

This is embarrassing, but I bought a used BS Air Mass II to use under my turntable which arrived without a manual -- I know, I probably should not need a manual, but I'm obviously doing something incorrectly. I placed my turnatable (Notiningham Interspace) on the Air Mass, and pumped it up -- it did not rise up evenly and the turntable rocks back-and-forth. Too much air I assume, but at no time in the inflating process did the Air Mass level off. So obviously I need help (I know, in more ways than one); any thoughts would be appreciated -- please be kind, at least to my face. Thanks.