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I have quite an old handpump that fits on my frame.

It looks like it's called TOPEAK "miniDUALG - optimised for MTB/ROAD."

It has a pressure guage and various bits that screw the handle to the body and the body to the end with the pressure/bar valve, with an interchangeable head that allows you to switch between Schrader and Presta / Woods/Dunlop valves.

Last night, when I got a puncture, it wasn't working. The pump was placing as much resistance to pumping in as pulling out (I may even have been drawing air back out of the tyre, I can't remember). Somewhere, there must be a blockage which is stopping the one-way valve from working.

But where is the air intake for the pump anyway? In the head (part of the valve adaptor), the middle (it has two sets of o-rings, with some cleverly designed stuff between them), or the handle (which has gaps in the screw thread down which air might flow from the outside — if that cup-shaped head seal would let it).

Anyone got any suggestions? I'm baffled - but I don't want to throw away a perfectly good bike pump if I can repair it.

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4 Answers 4

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I would first check the parts that ar exposed to outside, those that are connected to the tube valve. Something could be stuck in there. Check

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  • Thank you Vladimir. That's certainly the same pump (though my adaptor is blue). The other difference is that my adpator comes out with a small spring attached (very slightly bent askew). It appears to face downwards in the Schrader configuration. But should it swing round along with the adaptor and face upwards in the presta configuration? Or is it really supposed to remain inside the head? It occurs to me that it may have been obstructing airflow to the presta valve in some way if it was in the wrong position.
    – Eric Colvin
    May 18, 2020 at 13:47
  • @EricColvin the spring should remain inside the head, behind the schrader / presta adaptor.
    – Scottmeup
    May 18, 2020 at 20:42
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If I understand correctly you're feeling the same pressure on the compression stroke as the extension stroke?

The mini dual g pumps air during both compression and extension strokes so this would be normal behaviour https://www.topeak.com/global/en/products/pumps/239-mini

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    Oh! I hadn't realised. Thank you. Well that's absolutely the same pump. If I place my finger over the head-cap, though, it feels as though it is sucking my finger in on the extension (as opposed to trying to blow past it on the compression) — and the PSI guage goes high on compression and sucks back to zero on extension. Would that just be because it's my finger, and not a presta valve?
    – Eric Colvin
    May 18, 2020 at 13:39
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First of all try to use the pump without any tire attached to it. If it works fine, check that the tire valve is not blocked.

It might be that you have forgotten to unscrew the head, if there is a threaded cap, or the valve is stuck and you need to loosen it by pushing it down manually with the cap stem.

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  • Hmm. Without any tire attached, there is suction on the upstroke (extension) which took me by surprise.
    – Eric Colvin
    May 18, 2020 at 13:35
  • Correction. Whatever happened last night, it was resistance - but perhaps not suction. As Scottmeup pointed out, the pump is designed to pump in both directions. Fiendishly clever.
    – Eric Colvin
    May 18, 2020 at 14:18
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Thanks, guys.

I can't believe how swift and helpful were your responses (for such an ancient pump).

I think I may have it solved, thanks to all three of you. Won't know for sure until I get out to my bike and pump a tyre.

Firstly, by the looks of things in daylight, the PSI guage is now registering pressure in each direction. I had managed to pump in the end, last night (after loosening the screw-thread slightly at the handle and mid-section) — but, in the half-light of dusk, mistook the extra resistance on the upstroke to be suction; quite possibly, at first, the presta valve was obstructed or its seal not broken, so there would have been exceptional resistance.

That feature of pumping air into the tyre even on extension is really very clever. Does that mean, do you suppose, that there might be two different air intakes - one in the handle, one at the other end of the pump? Or is there just one at the front, with the two o-rings cleverly diverting air in opposite directions through some double ducted shaft?

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  • If there is an air intake at the front of the pump, where is is concealed?
    – Eric Colvin
    May 18, 2020 at 14:20
  • It's going to be tricky to say without taking one apart. I feel the simplest design would be to have one air intake feeding to a pair of opposing one way valves.
    – Scottmeup
    May 18, 2020 at 20:47

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