benjamin pump experts
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Topic: benjamin pump experts (Read 413 times))
rong1966
Plinker
Posts: 144
yes
Real Name: Ronald
benjamin pump experts
«
on:
February 03, 2019, 04:13:12 PM »
I recently replaced all of the o rings in my benjamin pump because it was taking way too many pumps to fill my marauder ft--over 250 pumps to fill from 1800-3000. After the rebuild it reduced the number of pumps to much less--just over half as much. The pump worked ok for a couple of fills before the handle would occasionally slip with no pressure on the downstroke. It just didn't feel right so I took it apart again. Now there is so much down pressure that I can barely fill the hose to open the valve! I used the kit from sun optics and ran into a problem; the o rings aren't labelled so I had to eyeball them against the chart that came with it. The o ring they list to use in the brass piece to which the black outer cylinder screws into (part 34 on their diagram which is a 118) is so thick that the middle cylinder won't fit into it. Also, the o ring they list for part 26 ( size 211) is too thin and is extremely loose. The parts however fit if I swap them but the pump which worked ok at first now has way too much pressure on the downstroke. Please help!
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Baton Rouge, LA
AlanMcD
Expert
Posts: 1367
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Real Name: Alan
Re: benjamin pump experts
«
Reply #1 on:
February 04, 2019, 08:15:42 AM »
I have no experieice with the Benjamin pumps, so I don't know anything about the rebuild kits or which o-rings go where. But I do know how three stage hand pumps work and they all work in a similar manner (my experience was with a Hill pump).
Study the illustrations in the link - I drew this up to explain how things work. Each stroke does two very different things in these pumps: the upstroke draws ambient air into the first stage and also transfers/compresses the air in the second stage into the third stage. The downstroke transfers/compresses air in the first stage into the second stage, and compresses the third stage into the line to your gun.
I am not sure I fully understand your description - if air is not getting out of the pump, but is getting into it, then it usually becomes either very hard to pull up (the handle wants to stay down) or the pump won't compress down (the handle wants to stay up). You say it won't put any air out, but other than that it sounds normal but difficult. Study the diagrams and think through the air flow and you should be able to figure out where your problem could be.
https://www.gatewaytoairguns.org/GTA/index.php?topic=145448.msg1477494#msg1477494
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Michigan
rong1966
Plinker
Posts: 144
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Real Name: Ronald
Re: benjamin pump experts
«
Reply #2 on:
February 04, 2019, 09:37:17 AM »
Sorry for the poor description, the handle is very hard/impossible to push down all the way after 3-4 pumps. I can't even use my full body weight. It will not pump enough times to fill the hose to open the gun's fill valve. It worked fine for awhile after I changed the o rings until the handle would have no resistance on the down stroke one or two times during the 50-60 pump session. That is when I took the pump apart again.
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Baton Rouge, LA
AlanMcD
Expert
Posts: 1367
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Real Name: Alan
Re: benjamin pump experts
«
Reply #3 on:
February 04, 2019, 09:48:32 AM »
OK - then I think the air is not getting transferred into the third chamber during the upstroke. If you look at the diagrams in the other link, you will see that the downstroke moves the air from the first chamber into the second one, and out the third into the hose. Since it is getting too hard to push down on only the third or fourth stroke, the air is not moving from the second chamber into the third on the upstroke.
Study that diagram on the airflow in the upstroke as it moves from the second chamber into the third. That is done by having an o-ringslide on it's gland to uncover a small air transfer port allowing the air to move between chambers - on the down stroke the o-ring blocks that port, but on the upstroke it is supposed to move out of the way to let the air by.
It sounds like maybe you have too thick an o-ring installed there and it is not letting the air past it on the upstroke. But do look for the air transfer path to see how it works and go from there.
Just a best guess - as I said I have not been in a Benji pump, and it has been a few years since I have been in my Hill pump (it is just a backup to my Shoebox now).
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Michigan
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benjamin pump experts