My primary floor pump started slipping off the presta valves on my bikes. A few years ago I serviced and lubed it to fix it, but this new problem was due to the chuck's internal rubber cylinder seal finally wearing out. My fix was to add spacers below the base of that rubber seal, between it and the chuck plate that pushes on it when you apply the lever to tighten the chuck. This increases the pressure that the chuck applies to the seal, which tightens it.
I knew this would work because I've fixed several pump chucks this way over the years. As it wears further you can simply add more spacers or use thicker ones.
Pictures worth 1000 words.
Here are the little spacers sitting on top of the chuck plate underneath the rubber seal. I made them by clipping off pieces of a zip tie
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Here's the rubber seal installed on top of the spacers
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