How does a pump push air into the tire and take in air from somewhere else? Why doesn't air spill out from the edges of the hole where the long thing goes in and out of the tube? I'm a complete noob to pumps.

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    I thought this would be a dupe, but its not. Well done for finding something everyone else missed.
    – Criggie
    Commented Jun 5, 2017 at 4:17

1 Answer 1


Someone made a nice video.

There is a rubber seal around the piston which makes it air-tight with the inside of the cylinder. Then there are two check valves which only let the air flow in one direction: one to allow outside air in when you pull (but not let is out when you push), and one to let the high pressure air inside your tyre (but not let it out).

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    And the inner tube has a valve which allows air in, but not out. Commented Jun 5, 2017 at 10:48
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    That video is neat, but it glosses over the valve operation. In practise, all you have is a curved rubber seal between the plunger and the pump body. The curve of the seal ensures a higher air pressure in the chamber pushes it tighter to the pump wall, so it stays sealed as you pressurise it. The higher pressure pushes the tyre valve open, and compressed air goes in. On the return stroke, the non-return valve in the tyre seals itself, and the resulting lower pressure in the chamber lets the seal collapse and air come in round the edges.
    – Graham
    Commented Jun 5, 2017 at 11:39

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