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Presta novice question. When removing the adaptor and quickly screwing the inside core down, air escapes. I've changed from a short Zefal pump to using a compressor. The tire says 50-60 PSI, but by the time it is disconnected I lose a lot of air. Do I go with the tire PSI or presta recs which are like 80-100? Thanks!

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    You should not have air escaping after the chuck is removed but before the knob is screwed down. If air is escaping at that point then there is something wrong with the valve. – Daniel R Hicks Jun 20 '16 at 23:25
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    The trick is to pull slightly on the core as you screw it down, keeping tension on the valve will keep it closed until you have screwed it all the way in. If you try and rush it you will accidentally push the valve in and open it, letting all the air out. – Lachlan Jun 20 '16 at 23:28
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    Note that the little knob should not be screwed down tight, just snug. If you over-tighten it you can damage the rubber gasket inside the valve. – Daniel R Hicks Jun 21 '16 at 0:04
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In my experience there is always some air loss when you remove the pump from a Presta valve, but that air is lost from the pump side of the valve.

It's easier to see with a floor pump, since there's the hose to hold air. In a floor pump there's a check valve is at the pump end of that hose, and that will hold the air in the hose at high pressure even after you let go of the pump handle. That high pressure air escapes when you release the valve.

With a handheld pump there should be much less air released when you pull it off, because there's probably no check valve - the path from the pump chamber to the tyre is so short it's not necessary. Because it's so short there's not a lot of volume to hold high pressure air. Even if you release the pump from the valve while holding the pump handle in, you'll hear a little "phut" as the air escapes rather than the long "pssshhh" from a floor pump. But usually you let go of the handle, it gets pushed out a little until there's not enough air pressure to push it further, and when you pop the handheld pump off the valve you don't hear anything.

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    Since the Presta valve closes after each pumpstroke the pressure in the hose is equal to the pressure in the inner tube. You will have to overcome this pressure with the following pumpstroke to get more air into the tube. When you remove the chuck from the valve the pressure is released with an explosive sound. That's normal – Carel Jun 21 '16 at 8:54
  • I was talking of floor pumps. Hand pumps are fine in an emergency and fiddly for most of the time, not a proper tool for building up decent pressure. – Carel Jun 21 '16 at 19:48
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You should never exceed the maximum pressure that the tire says. But the tube shouldn't leak any air when the Presta core is unscrewed before or after you connect the pump; at that point you should have to push on the little knob that you unscrew to make air come out. I did some web searching, and your problem with air leaking out of the valve seems uncommon. If that happened to me, I'd probably just buy a new tube. Good luck!

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    If you do web searches, it'd be handy if you pasted in the useful links you found so that the OP can read more about it. Even "this search {link} didn't find anything" at least helps the OP understand what terms you're using and maybe improve their search skills. – Móż Jun 20 '16 at 23:53
  • @Móż Fair enough! I searched for "presta valve leak" and found a discussion on a different web site speculating about dirt jamming the valve, or rubber seals inside the valve going bad, but that OP was asking about slow leaks, not fast ones. There was also a couple YouTube videos that didn't seem to be about the same problem either. – rclocher3 Jun 21 '16 at 0:01
  • Thanks for the info. Yeah, even negative results are often useful, and I think here they are. Coz the next thing the OP is going to do is that exact search... – Móż Jun 21 '16 at 0:25
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Since the Presta valve closes after each pump-stroke the pressure in the hose is equal to the pressure in the inner tube. The chuck does not mechanically open the valve as with Schrader (car-type) valves. You will have to overcome this pressure with the following pump-stroke to get more air into the tube. The maximum pressure obtained in the inner tube equals the maximum pressure the pump is able to build up. Track pumps have a valve between the cylinder and the manometer. The manometer will indicate and hold hold the value of the actual pressure.

When you remove the chuck from the valve the pressure is released with an explosive sound. That's normal. But you have to take care not to touch the sensitive top of the valve while removing the chuck because that will let some air escape. It's a matter of quick and precise movement.

Presta valves don't have rubber gaskets as opposed to Schrader. The airtight seal is achieved by a conical stem pressed into a conical hole. The little knob pulls the surfaces tighter. If the valve is in a good condition it will also hold the pressure without a tightened knob. If the valve leaks after disconnection from the pump the tube (or the core of the valve if it is of that type) needs replacing.

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Presta Valves, or any valve for that matter do not leak after you have taken the pump head off.
The valve should automatically close even without you having to screw down the valve. Doing that only applies pressure on the valve to reduce leakage over time even further.
If the valve keeps leaking till you screw it down means that there is some damage in the rubber of the tube valve. Where I live we can actually buy just the tube valve, disassemble the presta valve and replace the tube which might be damaged. However, that requires some experience and a tube valve which you are unlikely to find. Your best bet is to just buy a new tube.
Sidenote: Even on a new/fixed tube you will always have some very small leakage when you pull the pump head off. This is because that little pin that gets pressed down(that's what opens the valve and allows air in) when you put the head on has to come back up and that only happens as fast as you can take the head off. So don't be alarmed if you hear a short hiss when you pull the head off. To reduce it, try pulling the head in one clean jerk but it really wouldn't change much compared to the total volume of the tire

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