Crosman 760 Pumpmaster from 1977 has issues
Hey all. Just joined up to ask a question about my Crosman 760 Pumpmaster. Built in 1977, the number tells me. I bought it new, and it worked like a champ for decades. Just dug it out of storage, and when I pumped it 8-10x, the BB would not leave the pickup magnet. I just figured right away it needed a rubber cup etc so I bought to rebuild kit. New cup, 3 new o-rings, and new valve. Swapped everything out, and....no change really. SOMETIMES it will work correctly (only with 8-10 pumps) and it will hit a target, though not with much impact, it sounds like. But then, sometimes 8-10 pumps won't move the BB off the pickup again: just like before the rebuild. Sometimes it moves the BB, and I it's gone when I check, but....when I tilt the barrel up, the BB rolls back to the magnet: not enough oomph to actually leave the barrel. It's still inconsistent. Almost like the parts swap did...nothing. And...shouldn't I be able to shoot (although weakly) with...2-8 pumps? Right now, that's not the case.
Thoughts, other than "send it to a pro"? I called and they want more than it's worth, frankly.
Don't know a whole lot about the 760...have owned a few,and fixed a few.
Money-wise,folks love the3wood stock version (from about 1966-1975ish). There were some wood stocked versions procuced later than 1975, likely from left over parts or special oders.
Does seem that folks that got their airgunning started with them still seem to love them.
Were some rifled versions...those also seem to be move $-valued(although if you grew up with one as your first airgun,who gives a rat'sass about $ value?).
1.They aren't all that hard to reseal.
2. They do cost a bit of $ for someone else to reseal.
3. Are a little tricky to get right.
One test for just the cup/compression seal is to pump it slow vs. pump it fast. Speed seems to force the seal to expnd/compress some air, while slow gives a craked/worn seal time to eject /by pass the seal rather than compress it.
Pesoanlly,if I liked the rifle,I'd spend the $ to fix it.
IF I were concerned with $-in vs. $-out, I'd likely junk it if it were a typical smooth bore/plastic stocked version.
Then again, if I were younger and wondering about the 2030 sale p[rice...I might rethink that.
Further intel: If I pump the gun up rapidly, and immediately fire it, it will fire (with pretty good force) each and every time.
If I pump it up, then wait 10 seconds, it won't move the BB. This add anything to the mystery?
For cheap guns, always try the cheap solution first. Walmart sells automatic transmission stop leak, I think it's K&W brand. Supposed to rejuvenate worn seals. I've had good luck with it on fleamarket Daisy 880's. Put a few drops into the pump slot and cycle the gun a few times. (Tom Gaylord swears by this stuff for old pump guns.)
Saw your note about losing air pressure on a slow pump/aim/shoot, which seems to be what I have. But I have a new cup though. Ideas?
Really going to have to hunt down a rebuild kit based on the actual serial number/year of production as they did seem to change about over its production.
Can usually feel it when they are taking air..at least by the 4th or 5 pump you'd notice it getting stiffer to pump. When it get to that point,and you're pretty sure it has gulped some air, sit it in a corner for 10 min. or so and see if it still has air in it or not.
Get a schematic and make sure everything is installed correctly. If that has one of those cartridge type blow off valves the check valve isnt sealing. There is a small seal inside the pop off valve thats tricky to install. There are more than 3 seals there. https://www.ebay.com/i/291418181514?chn=ps&var=590519099038&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=590519099038_291418181514&targetid=595232421309&device=m&mktype=pla&googleloc=9018799&campaignid=2086169719&mkgroupid=76147899806&rlsatarget=pla-595232421309&abcId=1141016&merchantid=101706587&gclid=CjwKCAiAx_DwBRAfEiwA3vwZYsVsC3JEr7kvwwPORx5y8Uu3QoIDNYcdfffvvfxPocLWu2mt0rcB7RoCcjYQAvD_BwE
update: if I pump it and quickly hold the barrel to my ear, I clearly hear air escaping. OK, now...how to fix? What to look for?
See my post. Small seal INSIDE pop off section called a quad seal. If you didn't replace that one, that's probably it. You have to take it out with a pick and carefully roll it back in. You may have also nicked or cut a seal when you assembled.
Good luck. I have fixed a bunch of guns over the years that were not worth it monetary wise. If I do it for a friend and it puts a smile on their face, it is all worth it. I work for free though. It is just a hobby. Have fun.
Bob in WV
Still trying to figure out which seal you are talking about, Bill S. Do you have the schematic? I do, and don't see anything listed as a quad seal. Happen to have a part number so I can see what we are talking about? Also...pop off section? That ebay link you showed was for a 1976 and earlier version. Mine's a 1977 model. Don't think my innards look like that.
Did you inspect or replace the check valve (part # 1322-056)? It has to be it or the o-ring 130-034 or the exhaust valve 1322A011. If the check valve is brass with an o-ring on it, replace the o-ring. If it's nylon, check and see if there's any nicks on the sealing area (or debris in the sealing area), or if it's warped from being overheated from pumping air past it. What I do to check which part of the valve body is leaking is to blow air into each end of the valve body and see if the seal is holding, then you'll know which seal it is. If both valves hold air it has to be the o-ring sealing the two halves of the body together.
Inspect the valve seat area for the stem seal. The old seal was rubbet and the new seal is harder delrin or similar. The seat may need to be cleaned up. I also put the stem seal in and chuck the back end in a power hand drill. Then pulling on the stem and slowly rotating it with the drill to help it seat. Assuming that is an air sealing surface - I don't have a good picture of that particular valve. Either way, I'd be inclined to inspect the valve seat areas. Don't try to polish them though as they are delecate and don't contain much material.
Still stumped. Pulled it back down again, back together, still air escaping, and pulled it down again.
I opened the brass valve again, checked all the 0-rings, cleaned everything in alcohol inside and out, put the valve back together, and hooked up a compressor (with a rubber tip on the hose) and put it on the end of the valve. Wide open on the compressor got me...not a peep of air from the valve. All silent. Flipped the valve over and slid the hose over the end of the exit valve rod, slid it up to the valve and....silence. Not a peep of air at all. Hmmmm......
Bill S: I did not try your power drill seating method yet b/c you are correct: the stem seal is hard plastic delrin etc. There is no o-ring between the delrin pad and the brass seal tip end. You think the drill method could help? I'm thinking it would cut a slight groove into the seal. Think that would help?
I took a wooden dowel rod about the same diameter as the inside of the valve body and made sure it was square on the end and then glued a piece of emery cloth to it and made a valve seat squaring tool so I could make hard seals work on older model airguns. Most of the valve seats on older guns had imperfections that the old rubber seals would forgive, todays valve seals are too hard to use in the older valve bodies unless they were perfectly made. I would just turn the tool slowly by hand until I could see a shiny flat spot on the seat that was all the way around the seat, then I had a good flat seat for the harder seals.
Hey! I was just wondering how I'd try to clean up that seal face. It's not smooth at all. Not missing large chunks, but not smooth. I didn't think small pits in that contect edge would matter, but maybe they do?
I'll try this trick tomorrow. Sounds better than my idea of a Dremmel doing it.
Be careful - depends on what kind of a seat. If its already a flat surface, the dowel with fine sandpaper will work. Some models of valve seats have a small raised portion. You turn that flat it will never seal. I'm not familiar with the 760. Resealed countless guns but never one of those. Of all the stem seals I've replaced they all mated with a thin, raised seat surface. That includes guns like Crosman 140, 160, 180, 400, 600, Sheridan and Benjamin rifles and pistols, etc.
Well, I'm at a point where I need to try something else. The drill won't cut a "seal" into the Delrin: it's too hard of a surface. Barely scratches it. I did fine-sand the top of the metal seal edge that meets that Delrin stem. I could see that I made a difference in it, but, when re-assembled, nothing changed: still getting tons of air out of the barrel.
The O-rings on the brass valve are perfect, new, not nicked, and lubed. The cup is new and lubed. The Delrin valve is new.
The only thing left to try is possibly a new, old stock brass valve. Not sure what a new one would add, but I'm out of ideas. You?
I'll also add that I get the nostalgia factor of reviving the gun from your youth, but there is a reason most boys were envious of their friend's Sheridan or Benjamin pumper. Crosman 140/1400 is also a nice MSP rifle. Larger caliber (.22) rifled barrel, wood stocks and steel or brass barrels on those guns. Much more power and accuracy.
Just talked to Rick at length. Such a nice guy. Really knows his stuff in general, and knows my gun specifically. We went over everything that's been done, went over the parts diagram together, and he concurs: Originally the check valve was brass, and there was a urethane washer of some sort between them, right along that valve seat. When I took mine apart, it was brass-on-brass, no seat at all. That's why my seat is nicked up a bit. Even after sanding it, I don't think it's sealing well.
We both think a new complete brass valve (now I think they are AL though) will fix this. Rick didn't have one to sell, so back to ebay....will report back when its here...
Oh. I'll try Mike as well. Always better to buy from a referral.
The 1977 hammer drop is the same type of valve and cup as the current 760's, main differences are the new ones have the aluminum housing, and delrin stem cap. You can order the entitre valve assembly as a drop in from the crosman parts department. Part # is 417-101. Last one I ordered a year or so back was if I remember correctly was $6 + $4 shipping. If you need other parts the shipping has always been a flat $4 no matter how many parts are on the order.
The 1977-1983 760's are, IMO, one of the underated Crosmans. They are short stroke, striker fired, (hammer drop, manual cocked, etc,) have metal recievers, and use the same valve/pump system as the 13xx pistols, only real difference is the 13xx valve bodies have a screw hole for the front grip frame screw. A lot of mods you can make to a 13xx can also work on a 760, including flat topping, and lighter/heavier hammer springs, polished /fluted hammers, extented bolt probe, etc..
Some of the original barrels were rifled, or a rifled barrel and barrel support (breech block) Crosman part # 760E125, and a Crosman barrel part # MK177-010 (rifled) is a drop in combo and can be ordered from Crosman, last one I did was again a year or so back and was around $20 shipped for the barrel, and support. There are a few different stock options, an easy one is a Monte Carlo stock with cheek piece and longer LOP from a model 66, which is also a drop in part. The weak point on 760's is the lack of a bolt probe seal, and the transfer port seal can be leaky, but there are a few fixes for these also.
There used to be quite a bit of info on modding the 760's on the old Yellow, and the old Green Crosman forums, probably still in the archives.