Why does my hand pump feel like it's created a vacuum and won't pump?

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Dec 28, 2020
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St. Marys GA
I got my umarex origin, pumped it up to around 3k the first time non-issue other than I reached a point it was too tough to pump like I've heard with hand pump so I figured that's all she's got. I shoot the origin down to around 2k psi and haven't been able to pump any more air into it since, after the first few pumps it feels like its created a vaccuum and I can't pull it all the way up and if I release the handle it returns to the position where it feels like it starts to vacuum (20-25% on the upstroke).



Im completely new to pcp, looking online i see hand pump vacuuming but honestly I don't know what to search for and what I have hasn't helped me at all.



Does anyone here know what's going on, or atleast able to point me in the right direction so I can try to figure it out on my own?



Thanks
 
Its the pump that comes in the box with a new umarex origin. I used the provided silicone grease and lightly lubed the tube per instructions. I've literally only used it to pump the gun from empty to 3k once (not straight through, spaced out to not overheat pump) and the second time I try to use it im getting this vacuuming or whatever the proper term is.


 
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That symptom is most commonly caused by the final stage O-ring failing. The small O-ring on the innermost rod.

If you filled it all in one go from empty to 3k, you probably burned the O-ring. The longstanding recommendation is to limit to 50 strokes per session and give 15 minutes or so for it to cool down before continuing.

Since this is new, you have to make the decision whether to send it back or fix it yourself. Below I will explain how to repair it but I would not discourage you from contacting the manufacturer and see about other possible remedies. However just be aware the day will come when you will need to open it up so don’t send it back in the hopes you can postpone that day indefinitely. So if you have a modicum of mechanical aptitude, my advice is to open it up and fix it.

It’s best in my experience to replace it with a 90 durometer Disogrin polyurethane O-ring for longevity. Not standard 90 durometer polyurethane. These are sometimes referred to as “cast urethane”. Example https://www.theoringstore.com/store/index.php?main_page=index&cPath=367_1322

A Buna-N will do in a pinch but don’t waste your time with hardware store replacements because they are likely to be a lower durometer (softer) . I say “likely to be” because seldom do hardware stores list the durometer, but they are catering to stuff in the 20 – 150psi range where softer O-rings work better. Use them in an application like this and they will promptly extrude or wear out (abrasion loss). 90 durometer is a must no matter what the material.

When reassembling, lubricate everything with a thin film of 30W silicone oil. Not silicone grease…it’s too viscous. You can use grease sparingly on static O-rings to help with assembly, but any dynamic (moving) O-rings need silicone oil. The OEM oil that comes with most pumps is never labeled but you can generally assume it’s an appropriate silicone oil so feel free to use it if that’s what you have. Otherwise look for “silicone shock oil” on Amazon or at an RC hobby store.

Since you are dealing with a new pump there should not be any sludge buildup but if you do see anything of that nature, clean it up with paper towels and re-lube.
 
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That symptom is most commonly caused by the final stage O-ring failing. The small O-ring on the innermost rod.

If you filled it all in one go from empty to 3k, you probably burned the O-ring. The longstanding recommendation is to limit to 50 strokes per session and give 15 minutes or so for it to cool down before continuing.

Since this is new, you have to make the decision whether to send it back or fix it yourself. Below I will explain how to repair it but I would not discourage you from contacting the manufacturer and see about other possible remedies. However just be aware the day will come when you will need to open it up so don’t send it back in the hopes you can postpone that day indefinitely. So if you have a modicum of mechanical aptitude, my advice is to open it up and fix it.

It’s best in my experience to replace it with a 90 durometer Disogrin polyurethane O-ring for longevity. Not standard 90 durometer polyurethane. These are sometimes referred to as “cast urethane”. Example https://www.theoringstore.com/store/index.php?main_page=index&cPath=367_1322

A Buna-N will do in a pinch but don’t waste your time with hardware store replacements because they are likely to be a lower durometer (softer) . I say “likely to be” because seldom do hardware stores list the durometer, but they are catering to stuff in the 20 – 150psi range where softer O-rings work better. Use them in an application like this and they will promptly extrude or wear out (abrasion loss). 90 durometer is a must no matter what the material.

When reassembling, lubricate everything with a thin film of 30W silicone oil. Not silicone grease…it’s too viscous. You can use grease sparingly on static O-rings to help with assembly, but any dynamic (moving) O-rings need silicone oil. The OEM oil that comes with most pumps is never labeled but you can generally assume it’s an appropriate silicone oil so feel free to use it if that’s what you have. Otherwise look for “silicone shock oil” on Amazon or at an RC hobby store.

Since you are dealing with a new pump there should not be any sludge buildup but if you do see anything of that nature, clean it up with paper towels and re-lube.
Click to expand...

https://imgur.com/a/MzNPY9L which one of these would be the final stage oring you referred to? #11 ?



I'm going to tear it down either way,just trying to figure out what to look for.
 
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Not to dis agree but my hand pump has done that before . I would put a little pressure on the handle and open the bleed screw and it would start working again. I might have to do it a couple of times. I found that the handle has to be all the way down to start,then close the screw.

It scared me when it first happened and was much relived when I put my finger on the fix.

Hope this helps

God Bless

Bobby


 
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My guess is you needed a metric O-ring with a 1.5mm cross section. The -007 has a cross-section that is about 0.010" thicker. With softer O-rings, you can sometimes get away with it but the 90 durometer doesn't compress much. Unfortunately I am not aware of a source for metric cast polyurethane O-rings. Your best bet for something off the shelf would be a Buna-N in 90 durometer.

However I'll tell you what I would do instead. Take the -007 and stretch it over the shank of a drill bit, then load it into a drill press and get it spinning. Then spin up a rubberized polishing bit in a Dremel and slowly bring it in and just kiss the O-ring gently with it. Go slowly and abrade the OD of the O-ring down just a little so it fits.

Do a practice run with a throwaway O-ring to get a feel for it. It's worth the effort. Once you get the hang of it, it only takes a minute and it will bail you out of other inevitable metric/inch O-ring predicaments.
 
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