Benjamin Pump Assistance

[Mark Mowrey]

Alright all of you Benjamin pump gurus, I need some help.  I just got finished tearing down a Benjamin pump for cleaning and 007 o-rings replacement.  I reassembled the pump and it is not working properly.  The pump strokes are very inconsistent.  When I push the pump handle down, it raises back up by itself.  It also seems to be sucking air in through the fill hose on the up stroke of the handle.  Every once in a while I get a good pump stroke.  This is the second Benjamin pump I have "tuned up" and I did not have any issues with the first one.  The o-rings were not shredded in this pump like they were in the first one; I was half expecting them to be based on the pumps performance prior to the tear down.  It was a bit dirty inside though.  I ran the small parts through an ultrasonic cleaner and dried them thoroughly.  Everything was lubed with thin layer of silicone grease prior to reassembly.  I tore it down a second time with no better results.

What am I missing?

[nervoustrigger]

When you say that on the upstroke, it not worthy in air through the fill hose, I take it to mean you have nothing connected to the fill hose.  I think that's normal because the fill hose is not pressurized in this situation and therefore the check valve is not set. 

What happens when you connect it to a rifle or air bottle and try to pump it up?

As to the handle pushing up on its own, I had the same problem when using a hard (90D) -210 O-ring.  It doubles as a check valve by riding up and down in an oversized groove, and the hard material wouldn't let it move freely.  See my thread about it here:
http://www.gatewaytoairguns.org/GTA/index.php?topic=57170.0

[Mark Mowrey]

What happens when you connect it to a rifle or air bottle and try to pump it up?

When connected to my Marauder, the upstroke does not feel productive as in it doesn't seem to be taking in air.  The following down stroke results in no air transfer to Marauder.  After about 3 or 4 of these "empty" strokes, there will be one that seems productive, transferring air into the Marauder.  This is the same problem I was having before and the main reason for the rebuild.  I have tried using a quick, or forceful, upstroke but that did not seem to matter.

I only replaced the two 007 o-rings with 75A Durometer o-rings I purchased through Amazon.


Photo below: Old ring on the left, New ring on the right.  The old one wasn't in as bad of shape as I expected it would be.  Pump is about a year old.

[Mark Mowrey]

The factory 210 o-ring upon dis-assembly, prior to cleaning.  I will check this area again to make sure the o-ring is free to move.  This o-ring was NOT replaced.

[nervoustrigger]

...the upstroke does not feel productive as in it doesn't seem to be taking in air.  The following down stroke results in no air transfer to Marauder.  After about 3 or 4 of these "empty" strokes, there will be one that seems productive, transferring air into the Marauder.

Hmm, sounds exactly like the problem I was having with mine.  It was driving me nuts because I knew the behavior was like a check valve failing to seat properly.  So I focused on the plunger check valve in the bottom and ball check valve in the top but after checking, cleaning, and relubricating them, the problem continued.  Then I realized the -210 O-ring behaved the same way.  After replacing the hard version with a softer, more common O-ring, the problem went away.

[Mark Mowrey]

I still do not have the my pump working properly.  I replaced the 210 o-ring with a new 70 durometer one but that was not the solution.  The pump still moves up on its own after a down stroke even if it is not connected to the gun. It seems to be some sort of check valve issue.

This is driving me crazy...

I was recently reading the pump disassembly instructions on the Sun Optics site:
http://sunopticsusa.com/pump-instructions/

The fourth sentence from the bottom refers to Figure 10 in the photo below and states, "Be sure to keep track of the clear plastic washers (p-35)"

I do not recall seeing these washers upon my multiple disassemblies and I am guessing they came off when I ran the parts through the ultrasonic cleaner.  If they were clear, I would not have seen them in the funky water in the ultrasonic cleaner basin with a shiny steel bottom.  They would have certainly been dumped down the drain into the garbage disposal 2 months ago when this adventure began.

Does anyone have any input in regards to these washers: size, thickness, material...?

[Monkeydad1969]

Alright all of you Benjamin pump gurus, I need some help.  I just got finished tearing down a Benjamin pump for cleaning and 007 o-rings replacement.  I reassembled the pump and it is not working properly.  The pump strokes are very inconsistent.  When I push the pump handle down, it raises back up by itself.  It also seems to be sucking air in through the fill hose on the up stroke of the handle.  Every once in a while I get a good pump stroke.  This is the second Benjamin pump I have "tuned up" and I did not have any issues with the first one.  The o-rings were not shredded in this pump like they were in the first one; I was half expecting them to be based on the pumps performance prior to the tear down.  It was a bit dirty inside though.  I ran the small parts through an ultrasonic cleaner and dried them thoroughly.  Everything was lubed with thin layer of silicone grease prior to reassembly.  I tore it down a second time with no better results.

What am I missing?

Mark,

That gunk is causing the issue.  Remove that split washer completely and use 2 O-rings.  Don't go to heavy with the lube.  Call SunOptics to get the rebuild kit.  They will send it with the new white O-ring for that.  If they send 2 of the white O-rings, use them both.  I have had no issues for over a year with the new O-rings.

Joe

[bubba zanetti]

Mark:

Can't pull my pump apart for you at the moment, but the thin plastic washers have been replaced by a thin metal washer by some. Either in a video on You tube or the Marauder forum...can't remember  .

Also, the Sun Optics instructions only show one new white #007 O-ring going on the piston. So that's all I did and my pump is working fine. If we are supposed to use two I wish the directions were more clear.

And to add, Joe nailed it...DO NOT over lube the O-rings! Also, I used brake cleaner to clean off all that black goo. My pump was as bad if not worse than yours with the dirt and crud.

BZ

[Mark Mowrey]

That gunk is causing the issue.

I did't put it back together without cleaning it...

I just got finished tearing down a Benjamin pump for cleaning and 007 o-rings replacement...  It was a bit dirty inside though.  I ran the small parts through an ultrasonic cleaner and dried them thoroughly.  Everything was lubed with thin layer of silicone grease prior to reassembly.

[Mark Mowrey]

Mark:

Can't pull my pump apart for you at the moment, but the thin plastic washers have been replaced by a thin metal washer by some. Either in a video on You tube or the Marauder forum...can't remember  .

This gives me some hope and another lead to look into.  I have a strong feeling that the missing washers are the culprit; not allowing air to pass through the upper valve assembly properly.  Based on my measurements of the pump's upper valve, the outer diameter of the washer would be 0.5 inches and the inside diameter would be 0.25 inches.  Now I just need to know if thickness matters.  I will hit up the local hardware store after work and see if I can find something to try.

[Monkeydad1969]

That gunk is causing the issue.

I did't put it back together without cleaning it...

I just got finished tearing down a Benjamin pump for cleaning and 007 o-rings replacement...  It was a bit dirty inside though.  I ran the small parts through an ultrasonic cleaner and dried them thoroughly.  Everything was lubed with thin layer of silicone grease prior to reassembly.

There may be something in the small hole on the stem, behind the O-ring that may be clogging it.  That will cause the pump handle to rise up on its own.  I had the same issue.

Joe

[bubba zanetti]

Mark:

Can't pull my pump apart for you at the moment, but the thin plastic washers have been replaced by a thin metal washer by some. Either in a video on You tube or the Marauder forum...can't remember  .

This gives me some hope and another lead to look into.  I have a strong feeling that the missing washers are the culprit; not allowing air to pass through the upper valve assembly properly.  Based on my measurements of the pump's upper valve, the outer diameter of the washer would be 0.5 inches and the inside diameter would be 0.25 inches.  Now I just need to know if thickness matters.  I will hit up the local hardware store after work and see if I can find something to try.

That's it, the person re-placed it with a metal washer from the hardware store. Pretty sure it was in one of the multiple You Tube videos I watched before I pulled my pump down. Apparently the two thin plastic washers can be trashed somehow and the guy showed a metal washer and dropped that on top of the pump were the plastic ones go. They are pretty thin too.

You will get it up and going.

Heck, if I could then anybody can 

BZ

[Mark Mowrey]

I think I may have found the video.  If it is not the same one you saw, it at least addresses replacing the plastic washers with a metal one from the hardware store.  I will see if they have some brass shims first and if not, maybe look at using a stainless steel washer.

[crosman999]

 Make sure the ball valve is not sticking,its located under near handle.

[bubba zanetti]

I think I may have found the video.  If it is not the same one you saw, it at least addresses replacing the plastic washers with a metal one from the hardware store.  I will see if they have some brass shims first and if not, maybe look at using a stainless steel washer.

Thats the one 

BZ

[Mark Mowrey]

The ball valve is free to move.  I found a brass washer that seemed to be a perfect fit.  Pump still does not work.
Although this time I noticed some slop between the o-ring (part 36 in figure 10 of the photo above) under the washer and the main shaft (part 23 in figure 9).

 

[crosman999]

The white plastic washer that has a slit in it goes on last,I almost put mine back together with washer on first. Amazing that that tiny O-ring can handle that much work,no wonder that its the first to go...

[bubba zanetti]

Some of the spare o rings are similar but NOT the same depending upon the year the pump was made and a few extras are thrown in. Is it possible you may have the wrong one's in a few spots as above?

BZ

[Mark Mowrey]

The only o-rings I have replaced are the 007 and the 210.  I did not get a repair kit from Sun Optics.  I purchased a 100 ct bag of each from Amazon.com. The 007 rings are Viton 75 Durometer.  I apparently got 70 Durometer Buna-N on the 210 o-rings, but the only reason I ordered those was because replacing the 007s didn't fix the problem.  This is not the first pump I have repaired.  I got a used one with a used Marauder that I repaired and sold to my friend.  A simple replacement of the 007 ring was all that was needed and it is still working great.

I am absolutely stumped on this one. I have had it apart probably 10 times trying to troubleshoot this issue.
My pump was purchased new November 2012 in a Benjamin Discovery combo.

I am really considering the fact that it might be Hill MK3 time. This is absurd. It may be something simple that I am missing, but absurd nonetheless.

I would like to take the time to thank everyone who has commented on this issue and apologize if some of my responses seem crass.  Its just frustrating for an engineer when he can't fix a problem...

My split ring is on in the correct location.  I may replace it with a 007 o-ring and see what that does.  I expect nothing, I feel the issue is somewhere in the top valve assembly somewhere.  I might be wrong, but I think that when you push the pump handle down, it creates a vacuum between the black outer tube and the chrome one (14 in the Figure 9) to bring air into the pump.  Air is not being allowed to enter on the down stroke and the vacuum (negative pressure) created raises the pump handle to return to a neutral pressure.

[nervoustrigger]

I know this is an old topic but I was repairing one of these pumps recently and it would not take in air on the upstroke.  It's been a long time since I opened up a Benji pump so I just happened to take a look at Sun Optics' site and they had posted an updated manual somewhere along the way.  The pictures look like the same old poor quality ones they've always had but they now include a list of the O-ring sizes, and I noticed the -210 O-ring in the diagram above is actually supposed to be a -211.

As soon as I swapped it out, the pump started working like a charm.

Link to the manual: http://sunopticsusa.com/content/HP%20REPAIR%20INSTRUCTIONS.pdf