Author Topic: Crosman 760 Pumpmaster (1977 version) has issues  (Read 287 times)

geetee

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Crosman 760 Pumpmaster (1977 version) has issues
« on: January 13, 2020, 09:16:10 AM »
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.


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Alan

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #1 on: January 13, 2020, 09:40:24 AM »
Did you replace the o-rings in the valve??
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

geetee

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #2 on: January 13, 2020, 10:32:34 AM »
I did. There were two typical o-rings, and one small o-ring that as more like a "band" that was in the kit as well.  The parts list of the kit was:

(1) Valve Assembly Outer Seal (O-Ring)

(1) Valve Assembly Inner Seal (O-Ring)

(1) Breech Gasket/Transfer Port Seal (Square Ring/Gasket)

(1) Valve Stem

(1) Pump Cup

I just went out and tried shooting it again. Out of 5 BBs, all with 10 pumps on the gun, two exited the barrel at a somewhat lower velocity (I could follow them with my naked eye to the target, about 50' away. Three did not leave the magnet pickup. I remember never even being able to see it fly with 10 pumps!
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geetee

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #3 on: January 13, 2020, 11:51:18 AM »
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?
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Alan

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #4 on: January 13, 2020, 01:08:49 PM »
Something is leaking! Sometimes, as careful as a guy can be, you still cut or damage an o-ring. I've been there so many times....!

Make sure the gun is EMPTY, and check it twice!. Pump it up rapidly, and listen at the end of the muzzle and/or the entrance to the pump, to see if you hear the leak. You might not be able to hear it, but if it is leaking as fast as you indicate, you will hear it. If you do, it will at least narrow down the area.
  • Roswell, New Mexico
Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

Capt45

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #5 on: January 13, 2020, 01:09:47 PM »
Sounds like a leak somewhere.  I'm the master of the obvious, OR, I don't have a clue.
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geetee

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #6 on: January 13, 2020, 02:01:26 PM »
Alrighty then....I'll tear it down again tonight and report back. Thanks for the tips.
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geetee

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #7 on: January 14, 2020, 05:25:41 AM »
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?
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Alan

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #8 on: January 14, 2020, 06:44:01 AM »
VALVE!

I have taken apart just one 760. From experience, what little I have, I'm not surprised as the o-rings are really easy to damage when inserting.
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

steveoh

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #9 on: January 14, 2020, 01:06:33 PM »
VALVE!

I have taken apart just one 760. From experience, what little I have, I'm not surprised as the o-rings are really easy to damage when inserting.

Grease those o-rings up with silicone grease before reassembling!
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geetee

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #10 on: January 15, 2020, 10:28:02 AM »
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......
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geetee

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #11 on: January 16, 2020, 07:43:59 AM »
Well, I'm at a point where I need to try something else. The idea of using a drill to pull/grind a "seal" into the Delrin didn't work. 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?
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geetee

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #12 on: January 21, 2020, 10:02:54 AM »
New valve coming today. Should know more soon.
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geetee

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #13 on: January 23, 2020, 06:53:54 PM »
Put a new valve in it just now, and...no leaking from the barrel end! Have not shot it yet (no indoor range), but just pumping it and pulling the trigger sure sounds and feels like old times! Not sure what was wrong with the old valve, but if yours is giving you trouble, just start by buying the complete valve and cup, and not the valve rebuild kit. You'll save $ and time in the long run. Wish I had done it that way.

I did notice that when you pump it 10X ( I think 10 is the max, right?), it doesn't get progressively harder to pump. Seems like I remember that happening when I got this in my teens. Could be remembering it wrong though...

I DO recall pumping it a LOT more than 10x as a kid though. Like 50 or more, thinking "more pumps= more power". That probably contributed to the valve failing I bet.
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steveoh

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Re: Crosman 760 Pumpmaster (1977 version) has issues
« Reply #14 on: January 23, 2020, 07:34:48 PM »
I had a leaking valve on my custom .257 Citrus. Chucked it in my drill press and gently shaved the surface of the valve with a new utility knife blade. Put it back together and it sealed!
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