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Benjamin Hand Pump Repair

December 26 2010 at 11:49 PM
Mach-1  (Login Mach-1)

Turns out the check valve spring failed. It rusted and broke in two. Got a new one from SunOpticsUSA and slapped it in. Also got a complete seal set. But the old seals looked OK so I reused the old seals.

Cleaned the guts with Brake Clean (AutoZone brand brake cleaner) and pumped the liquid out of it. Lots of black greese and what looked like bits of metal came out. left it open to dry then poured in some Royal Purple 10w-30w motor oil to relube it.

Put it together and cycled the pump a bit to spread the oil. The pumped it to 2,000psi several times and then bleed it off to dump the excess oil. Not much came out. Used my Marauder for this task and was careful to not to let the pump pressure get high enough to pass air to the rifle. Then I topped of the Marauder to 2700 psi.

Easy pumping - seemed easier than when the pump was new. It pumps very easy up to about 2200psi and getting the last 500psi is not too bad. For now this seems like a nice easy fix for my broken hand pump. Took about two hours including drying time.

SunOpticsUSA suggested using whatever automotive motor oil I had on hand. Royal Purple is what I had on hand.

What say you - are either of these chemicals bad for the pump?

This message has been edited by Mach-1 on Dec 27, 2010 9:15 AM

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(Login USMCShooter)

Pretty much what I did with a buddy of mine's about 3 weeks ago...

December 27 2010, 7:55 AM 

But his was just severely gummed with a blown O-ring....he'd been using it for 3 years straight and it quit on him.....I brought it over to my house and tore it down, sprayed it all out with brake cleaner...let it dry, replace the O-rings with 90 durometer.....lubed it all with Moly....put it back together and it's up and running great....even smoother than factory....

These little pumps get a really bad rap....but, I've done two FX pumps and 4 Hill pumps for friends.....the problem with them is cooling time....guys don't pump slow enough, and don't allow them to cool down in between fills.....

I think they're the best value for the money....My Son's has over 1500 fills through it, to 32-3400 psi in various guns.....



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Bill S.
(Login tripleguy)

I would have used non-detergent motor oil

December 27 2010, 10:35 AM 

Generally, a multi-vis oil is going to have detergents added which will cause the rubber o-rings to swell and fail. Use a straight 30 weight oil or auto trans fluid.

"but I'll be needin' that gun, fer squirrels and such."

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(Login Mach-1)

Detergent? Good thought.

December 27 2010, 3:11 PM 

Thought of that too asked SunOpticUSA and the answer was oil with detergent would be good for the pump.

Thought seriously about Type-F transmission fluid. But went with the 100% synthetic oil. Thinking synthetic would be OK. Was more concerned about the brake clean.

I have a new set of seals in my tool box just in case.

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(Login Springer_177)

Engine oil and brake fluid cleaner are compatible with the pump o-rings

December 27 2010, 11:00 PM 

Some don't like using engine oil as a lubricant because it is a hydrocarbon and compression-ignition could cause detonation. The conditions do not exist for this to happen, but some are extra-cautious.

Most 'o-ring lubes' are a mixture of silicone grease and lithium grease. I think this is to make them compatible with a wide range of elastomers. The Buna-N (nitrile) and urethane (polyurethane) seals are completely compatible with engine oil, both detergent and non-detergent, synthetic, and non-synthetic (and auto-transmission fluid).

Just remember to keep the pump cool by doing 50-75 strokes every 20 minutes. Keeping the pump cool does two things: it keeps the seals from getting hot and deteriorating faster and pressurizing air causes condensation that will collect in a cool pump and can be expelled through the bleed valve. Ignoring the 20 minute rule will result in premature pump failure and moisture carried downstream of the pump (i.e. into the air rifle).

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paul self
(Login ranedouglas)

benji / air force footpumps

December 28 2010, 5:52 AM 

i repair them for the guy that invented them. use the newer o-rings. they are a better material. use 30wt motor oil. non-detergent if you can find (but it was hard for me to find). you should clean the pump as you clean your gun - every so often. there's a full guide to refurbing the pumps yourself online and complete reseal kits from sun optics for about $6. i'll try to get more info posted on my website sooner or later. - paul d self. http://raneair.webs.com .

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