Benjamin 312 issues.

[RobertMcC]

I picked up a 312 couple weeks ago ( no serial number so I believe pre 1957 ) and Its been hitting hard 650-700 FPS at 9 pumps with 14.3grn pellet.

I was doing a avg FPS per pump test today. 3 shot per pump number. I completed number 4 and moving on to number 5. Than there was little resistance in the pump arm and FPS dropped down to 275 FPS range In order to get it to shoot 500 FPS again I need to pump hard and fast.

Does this sound like a valve seal that going?

[bbv13]

Is it still holding air over night or longer? If so it could be the pump cup otherwise it's the valve. I guess it could have some trash in it that could possibly be pushed through it by lubricating it then pumping and shooting it a lot?

Is air coming out the end of the barrel when you try to pump it up?

Bryan

[RobertMcC]

Sometimes you can air hissing after you pump, sometimes you don't.. Sometimes it hard to pump ( building pressure ) sometimes its not.

When I first got it, I took it apart to clean because was a lot of crud. Everything except the valve. Lubed the pump cup and that was it. I shot maybe 100rd it since.

Almost sounding like need a detail clean and reseal.

[bbv13]

It sounds like it may have some trash in the valve. I recently read in the Sheridan repair manual that a 50/50 mix of 30wt non-detergent motor oil and Marvel Miracle Oil can be used after a reseal. I assume this is to break down crud that could cause a problem not as a every day lubricant. Worse comes to worse you may want to give it a shot before doing a reseal.

Lots of good info here. I'm sure some others will chime in and many of them know more than I about these old guns.

Bryan

[RobertMcC]

Yeah there was oily dirt on the piston linkage, compression tube, bolt etc. Almost look like carbon after you fire a firearm. I could only clean it soo much being I did not have the tool to take apart the valve.

[Rob112o]

If it's giving you resistance, more then normal, when you try pumping it. It's something keeping the check valve from moving back and letting the air in. Most likely some crud small debris chunk of oring wedged itself between the check valve and side wall of the rifle. If you are hearing air coming out of the barrel while you are pumping. That's the good ole check valve oring. I've come to notice that one is usually always the problem if your rifle isn't holding air for extended periods of time. Now, based on how old it is it's probably dunzo and needs new seals. It was firing good for a little while because the lube was sealing the pressure. Now that you fired most the lube out it probably moved crud around in the valve body. With that air pressure going thru it, it probably took a chunk out of some of those old cracked orings. How hard was the pump cup? Those old school ones tend to hold up better. So I wouldn't be surprised if you told me it was still soft and reusable. You are gonna have a single ported tabbed valve body with tabbed lead oring too. Those are a pain in the butt. I'm currently working on a 61" 317. With yours being older it's probably gonna be worse than mine inside the body.

[RobertMcC]

Still soft and usable.

What you mean pain in the butt?

[bbv13]

There are no o-rings on the outside of your valve because the valve body is soldered in and cannot be removed. If you have to reseal it you will need a valve tool to take it apart. Most guys that sale reseal kits also sell the tool so you can order them together. The lead seals are not that much of a problem. You just have to make sure it's lined up right. There's a really good video on YouTube about resealing the 312 that you can reference as you do the work.

Bryan

[RobertMcC]

There are no o-rings on the outside of your valve because the valve body is soldered in and cannot be removed. If you have to reseal it you will need a valve tool to take it apart. Most guys that sale reseal kits also sell the tool so you can order them together. The lead seals are not that much of a problem. You just have to make sure it's lined up right. There's a really good video on YouTube about resealing the 312 that you can reference as you do the work.

Bryan

Guess I have to order from Baker. None of the CDN dealers can get me the parts.

[Rob112o]

I just ordered a kit from Mac1. I let him know it was the single ported tabbed valve one.
It's a paint in the butt compared to a non-tabbed one because you got to align the valve and, big AND, tabbed lead seal together on the notch in the tube body. Now think about it, looking down into the small tube and trying to drop valve stem, valve, lead and valve nut perfectly. Fighting resistance from spring while trying not to turn or rattle the tube slightly, cause if you do that the lead ring or valve moved slightly off alignment, while you are trying to thread it. Whereas newer ones you just put all those parts on the end of The tool, slide in, compress spring, turn clockwise till you feel thread drop, turn clockwise, BOOM dunzo.

[bbv13]

If you have some Marvel Mystery Oil give it a soak over night just to see if it helps? All the guns I've bought recently work. I'm going to try it next time I buy one that's gunked up. There's nothing to lose if your going to reseal it anyway.

Bryan

[RobertMcC]

I just ordered a kit from Mac1. I let him know it was the single ported tabbed valve one.
It's a paint in the butt compared to a non-tabbed one because you got to align the valve and, big AND, tabbed lead seal together on the notch in the tube body. Now think about it, looking down into the small tube and trying to drop valve stem, valve, lead and valve nut perfectly. Fighting resistance from spring while trying not to turn or rattle the tube slightly, cause if you do that the lead ring or valve moved slightly off alignment, while you are trying to thread it. Whereas newer ones you just put all those parts on the end of The tool, slide in, compress spring, turn clockwise till you feel thread drop, turn clockwise, BOOM dunzo.

I love to order the Mac1 seal kit, but I also need the tool which he doesn't sell. So that's a extra 15-30 USD to ship it. I'm Canadian and our dollar is &^^&.

[Ted Howe]

FYI, I sent my 312 to Dennis Baker in Ohio and its shooting 890fps with 10 pumps.
Baker Air