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Hitching a ride
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Discussion Starter · #1 ·
It's a very well made pump but it lets a lot of air out of the shock when you unscrew it. I'm not talking about the air squirting out of the hose. I'm talking about the shock losing 15psi even if you unscrew as fast as you can. If you pause mid way on the un-screw it will drain the shock.

I guess they made the internal nubbin too long. At any rate, it's almost useless.
 

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Trying to measure the pressure in a shock will always result in reading a pressure lower than what was in the shock. When you put the pump on to check the pressure, air leaves the shock to pressurize the hose and gauge and in so doing reduces the pressure in the shock, often 10-20psi depending on the relative volumes and pressure. Normally, when you remove the chuck from the shock, the valve in the shock closes before air escapes from the hose and pump, so the pressure in the shock remains at what you set it. The air you hear leaving is only that from the hose and pump.
 

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just hog out the inner part of the pump chuck. I have an old shock pump like this, and it's now wide open in there. need to fight the schrader spring to get air moving which is like 2-3 pumps

if you need to pressure down or adjust down, remove pump, bleed by hand and re-pump up to what you want. My schraders open at 50 or so psi, which is fine since my target is either 115 or 220 depending on shock or fork. anyhow dremel the guts as you said or just drill out the nubbin and make it open
 

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Hitching a ride
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Discussion Starter · #8 ·
Trying to measure the pressure in a shock will always result in reading a pressure lower than what was in the shock. When you put the pump on to check the pressure, air leaves the shock to pressurize the hose and gauge and in so doing reduces the pressure in the shock, often 10-20psi depending on the relative volumes and pressure. Normally, when you remove the chuck from the shock, the valve in the shock closes before air escapes from the hose and pump, so the pressure in the shock remains at what you set it. The air you hear leaving is only that from the hose and pump.
It's all clear now. Lock the thread, mods, this one is taken care of!
 

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just hog out the inner part of the pump chuck. I have an old shock pump like this, and it's now wide open in there. need to fight the schrader spring to get air moving which is like 2-3 pumps

if you need to pressure down or adjust down, remove pump, bleed by hand and re-pump up to what you want. My schraders open at 50 or so psi, which is fine since my target is either 115 or 220 depending on shock or fork. anyhow dremel the guts as you said or just drill out the nubbin and make it open
Maybe I'm missing something here, but if you're using the pressure in the line to open the schrader valve won't that same pressure keep the valve open as you unscrew the line? You'll still have pressure loss, just less than if the pump's fitting holds the valve open mechanically.
 

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^ no. Schraders have a spring. If the pressure in the pump and the shock is the same, the valve will closes due to the spring force as the chuck is unscrewed and before air is allowed to escape. When the chuck is fully screwed on, there's a pin in it which opens and holds open the schrader valve in the shock. Air can flow both ways, the pressure in the shock is the same as in the pump hose and gauge. As the chuck is unscrewed, the pin retracts and the valve closes before air is let out of the hose and gauge.

When working properly, this is how all shock pumps and shocks work.
 

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a proper shock chuck with accurate length nubbin is most accurate reading.

the open chuck idea...the first hiss when unscrewing the open line, it will be closed. it always wants to close, and while you are pumping watch the needle. needle is accurate if you are pumping air in. it still takes XY pressure above current pressure to open the valve again for the next pump...so it's fiddly when there is no nubbin holding the valve open for ya...but works. the point is no air loss but some risk of a few psi too much

in OP case the nubbin inside is too long so when the first hiss comes, valve is still slightly open, farting all the stoke out of the air chamber
 

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Here's a simple test. Pump up shock and then slowly start unscrewing the chuck until you just start to hear air coming out. Leave it there until it stops, then take the pump off the shock. Has all of the air been let out of the shock? If it has, something aint right. If the shock is still firm, then all is good.

Some chucks have a lever or other means of separately actuating the schrader poppet. The Bontrager shock pump has a lever and the Specialized floor shock pump and a coaxial knob to do that. With these, you screw it on, flip the lever or screw the barrel down, pump, unflip/unscrew to close the schrader, and remove the chuck.
 

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Here's a simple test. Pump up shock and then slowly start unscrewing the chuck until you just start to hear air coming out. Leave it there until it stops, then take the pump off the shock. Has all of the air been let out of the shock? If it has, something aint right. If the shock is still firm, then all is good.

Some chucks have a lever or other means of separately actuating the schrader poppet. The Bontrager shock pump has a lever and the Specialized floor shock pump and a coaxial knob to do that. With these, you screw it on, flip the lever or screw the barrel down, pump, unflip/unscrew to close the schrader, and remove the chuck.
This is the correct answer. 99% of the time there is no issue with shock pumps and this "issue" is over blown. There are a handful of pumps and valves that do cause issues. Lone rangers test is the way to figure out if you are part of the 1% or the 99%.
 
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