since this isnt an industrial grade thing, maybe you could use a small plastic billows with a linear solonoid, I cant think of a specific place you could get them but you see them on toy launchers that you pound down on to launch, and you see them in drugstores.
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and ones like these are sometimes inhardware stores
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attach a small linear solonoid like one of these
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The down side is that it would apply suction once, then it would slowly leak it over time, but this probably wouldent matter for the length of a step. Also you would need a different one for each group of feet that step at the same time. But all in all I think it would be lighter and cheaper than a vaccume pump, and possibly suitable to your use.