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Losing air right after pumping

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Losing air right after pumping

Old 09-07-07, 07:57 PM
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remixity
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Losing air right after pumping

When I release the head of the Topeak Joe Blow Pro from the stem, I always lose about 5-10psi because the valve doesn't close right away. I have to compensate by pumping 125-130psi to get around 120psi.

I simply pull the pump head up off the Presta valve. Is there a better way to do this?
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Old 09-07-07, 08:06 PM
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That's excess air discharging from the pump itself. You're not losing any air through the presta valve as you pull the chuck off.
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Old 09-07-07, 08:10 PM
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You sure?

The presta valve is a simple poppet valve. The internal tire pressure normally holds it closed.

When you inflate the tire you actually pressurize the air in the hose until the air pressure in the hose exceeds the tire air pressure enough to push the valve open. When you release the hose from the presta valve, almost all of the air that you hear escaping is from the pump hose. As soon as the hose air pressure drops below the tire air pressure, the valve is pushed tightly shut much more quickly than you'd ever be able to do by hand.
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Old 09-07-07, 08:18 PM
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its the air held in the hose.. and even if it wasnt.. whats the big deal?
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Old 09-07-07, 08:34 PM
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Just pump it up 5 PSI more than normal, then when you pull it off it's back to where you want it. Problem solved...
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Old 09-07-07, 09:06 PM
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I'll check the valve again tomorrow to pinpoint the sound. Could've sworn it came from the valve and not the chuck.
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Old 09-07-07, 09:24 PM
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How do you tell what your pressure is after you've removed the pump head???
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Old 09-07-07, 10:07 PM
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I stuck it back onto the valve to recheck pressure. I've been getting inconsistent readings, could be the pressure indicator playing tricks on me.
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Old 09-07-07, 10:15 PM
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Originally Posted by urbanknight View Post
How do you tell what your pressure is after you've removed the pump head???
this is what I got...works well and has a relief valve as well




getagauge.com
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Old 09-07-07, 10:45 PM
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Originally Posted by remixity View Post
I stuck it back onto the valve to recheck pressure. I've been getting inconsistent readings, could be the pressure indicator playing tricks on me.
That's the problem. The indicator on the pump isn't meant to be used alone. You have to pump a few pumps (to depress the valve) before it can give you a reading, or to start filling the tube for that matter. So if you just stick it back on, it will give you the pressure between the head and the valve, not inside the tube.

Originally Posted by Pharmr View Post
this is what I got...works well and has a relief valve as well
Yeah I know about those, but I wanted to know how the OP was checking it. As I suspected, s/he was trying to use the pump, which won't work.
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Old 09-07-07, 10:50 PM
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Originally Posted by urbanknight View Post
Yeah I know about those, but I wanted to know how the OP was checking it. As I suspected, s/he was trying to use the pump, which won't work.

nevermind
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Old 09-07-07, 11:30 PM
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Still a good suggestion for the OP, though, if s/he wants to check like that.
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Old 09-08-07, 05:17 AM
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+1 I'm late into the thread, but if you pump to pressure, and remove the chuck, the pressure in the tire is correct.
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Old 09-08-07, 05:23 AM
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If you stuck it back on to get a reading then yes you are definitely going to get a lower reading, because
now you've let even more air out. The initial hiss you hear is coming from the pump as stated above.
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Old 09-08-07, 04:51 PM
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Soon you will realize that you worrying way to much about this.. and you'll just start pinching your sidewalls with finger and thumb to see if its full enough to ride, and you'll get on with your training....
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Old 09-08-07, 06:16 PM
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One of my Presta, long stem, smooth valves sticks in the open position some times. If this happens, you can loose a lot of psi before you can "bump" the head to get it closed.
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Old 09-08-07, 07:59 PM
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Originally Posted by Terex View Post
One of my Presta, long stem, smooth valves sticks in the open position some times. If this happens, you can loose a lot of psi before you can "bump" the head to get it closed.
Well then, get rid of that tube.
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Old 09-08-07, 08:19 PM
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Originally Posted by remixity View Post
I simply pull the pump head up off the Presta valve. Is there a better way to do this?
Would using a schrader adapter help? I had to get one because the short stem prestas on my new tubes aren't long enough for my pump, and it seems like disengaging the pump is a bit cleaner w/ the adapter.
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Old 09-08-07, 08:43 PM
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Originally Posted by waterrockets View Post
+1 I'm late into the thread, but if you pump to pressure, and remove the chuck, the pressure in the tire is correct.
It's most certainly close enough LOL
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