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@moth
^How did you find the build quality on yours? I bought one a while ago and my impression was that it's a genius idea let down by cheap construction. The finish on the aluminium middle cylinder was disappointingly rough and high-friction against its seals, while mould lines on the inner plastic plunger/pressure gauge caused its seals to leak, and the valves between the pressure gauge and the inner cylinder just don't work, so the gauge doesn't function and the pump relies on the tyre's valve instead, loosing some more efficiency.
Which is a real shame as it's a really good idea for a pump - the 'in' stroke doesn't force all its air into the tyre but instead compresses it into two thin annular spaces between the plungers and their corresponding cylinders. This effectively takes away the hardest last bit of the 'in' stroke and spreads it over the whole of the 'out' stroke, allowing the pump to be both high volume and high pressure at the same time.
Valve stem damage should be a lot less of an issue with this scheme as it avoids the jerky high-force bit at the end of a conventional pump stroke.