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My tyres are rated at 120psi and I like to inflate them close to that. My pump is electric with presure indicator. If I just unscrew the pump and connect it back the pressure drops by about 20psi. I have tried multiple pumps with the same result on both wheels.

Is there any device that I can connect between the pump and the valve to allow a quicker / 'cleaner' disconnection from the valve? Of course I can push to 120 psi and let it drop, but trying not to get close to the limit.

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    I have to say most Presta adapters suck. It's better to have a Presta chuck, or one that is "agnostic". – Daniel R Hicks Dec 6 '17 at 3:53
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    The limit basically means nothing. If you want 120 psi and go a bit over and let the air down, itll be fine. – Batman Dec 6 '17 at 5:31
  • A solution might be to run presta tubes with a grommet in the rim. – Chris H Dec 7 '17 at 12:41
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    "unscrew the pump" - I believe this is the biggest problem, a screw fitting. If possible, you might want to switch to a pump that uses a clamp instead. It results in very little air loss. – nogasbiker Dec 11 '17 at 20:17
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My current pump (a floor pump) clamps onto the valve with a clamp instead of threading it on. I can very quickly release it. This could be at the expense of not being able to pump it to 120 PSI (which I don't need).

It may be possible to adjust the pin in the business end of the pump so that it stops pressing on your valve sooner when you unscrew it; that way you lose less pressure.

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    My floor pump (probably 15 years old) clamps onto a Presta stem without screwing and releases with very little air loss. I regularly run 100 psi and could easily get to 120-130 if I wanted to (and didn't wear myself out pumping). – Daniel R Hicks Dec 6 '17 at 3:55
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    Mine wasn't adjustable. Ended up filing away good 1/4 inch and now I don't have to turn it so much. Still need to smooth out the edge because it is a little harder to turn now. Tks. – Aelian Apr 9 '18 at 5:47
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Much of the drop you see will be due to the reconnection, not the disconnection.

When you disconnect the pump you have 120psi in the tyre and the hose, gauge and a little bit of pump. This immediately drops to zero (as these are all gauge pressures). Most of the hissing you hear is this air leaving the system

When you reconnect the 0 psi in the hose etc. is averaged with the 120 psi in the tyre, weighted by their volumes (a slight approximation but it will do).

You can test this as you have Schrader. You can get a little tyre pressure gauge from a car parts shop that has very little volume inside (do the experiment at at lower pressure if it doesn't go to 120psi). Pump up the tyre. Measure the pressure with the gauge a few times, nothing the pressure each time. Connect and disconnect the pump. Measure with the gauge again. Repeat until you're convinced.

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    Thanks. Didn't suspect Boyles Law. Will test and report back. – Aelian Dec 7 '17 at 2:59
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    At least in my case this seems to be not the problem. 1) Tried with a small gauge and almost 15psi drop is during disconnection. 2) Tried on a different bike with a presta value and I don't have the problem. Just a quick pop during disconnection. (Late update due to long winter.) – Aelian Apr 9 '18 at 5:44
  • @Jeevaka fair enough. Some valves get a little sticky over time, and there is a little variability in effective pin length, so you wouldn't necessarily expect them all to behave the same – Chris H Apr 9 '18 at 8:47

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