Air Gun Home :: View topic - Why is air leaking out here? (Crosman Powermaster 760)

themdg · New Member

Hi There. Great forum!

I was given my grandpas old Crosman Powermaster 760. We put 50 or 75 BBs through it with no problems. Shoots awesome...powerful with few pumps.

After that, it quit holding air pressure. I disassembled it to check it out (and to clean it), and found that air was leaking here:

I don't know anything about how the air storage system system works. Can someone else with more experience provide some info?

Here is a shot of all the parts, in case it's helpful.

Thanks in advance.

Doc Sharptail · Member

Could be one of a few different things. You'll have to get the valve out of the gun, and dis-assemble it to find out. Take the roll-pin out of the front of the pump tube- on the older guns, they seem to come out easier left to right. Once the roll pin is out, remove the tube end plug, the pump linkage, and pump cup.

Remove the rear cap pin, and rear tube end cap. The hammer/spring assembly can now easily be removed. You will need a long dowel or similar to get the valve out- it is pushed out the front of the tube....

http://www.crosman.com/pdf/manuals/C760SC-EVP%20&%20PL.pdf

That should help as a bit of a guide...

Regards,

Doc Sharptail

themdg · New Member

Thanks, Doc Sharptail. That exploded view will help a lot.

It looks like the back pin pushes out, and the front pint has some sort of locking nut or washer that should be removed first, is that right?

I'll try and get that assembly apart tonight and report on my findings.

edit: Shoot...should I have posted this in the "Maintenance and Modification ->Rifles" forum? Maybe I blew it.

Doc Sharptail · Member

Yes, that little cir-clip is sort of like the grand-dad of the e-clip. Use a small bladed screw driver to get one end of it lifted up, and rotate it off the pin. It took me a few tries to get it figured out. I think your post is okay here- if not, it will simply be moved with a tag left on this page...

Regards,

Doc Sharptail

themdg · New Member

Thanks again for the disassembly tips. I was able to pull it apart without too much trouble. Turns out both of my pins were able to be removed with only pushing. I read something in the exploded diagram about clips being added after a certain year. Might help me date this little thing.

Everything was pretty dirty, but cleaned up nicely. I see now that the screw hole I was pointing to has nothing to do with releasing air...it was just the closest escape for the air.

On the parts I pulled out, none of the metal looks fatigued or misshapen. The plastic and aluminum parts look ok too. I see 3 rubber o-rings (well, 2 looks like roundy o-rings, one looks more fitted...the one still on the large brass part.). While the rubber looks good and healthy, and I see no cracks or stretching...I suspect this is likely my problem anyway. I'm posting some pictures so anyone can have a look. I'll post a few extras so all the parts are captured here. Maybe it'll help someone else with another problem some day. Below these pictures are links to high-res versions...in case anyone wants a CLOSE look.

What's the general recommendation on these parts? Find exact replacement o-rings and replace all 3? Use some rings from my little bag of plumbing rings? Dry the exsiting rings off nicely? Oil them up ? (I've been using Rem-oil to clean...) Or maybe the common culprit is not the rings at all...

I welcome any further light.

http://themdg.org/pics/posts/2010-06-08-BrokenBBGun-05-large.jpg
http://themdg.org/pics/posts/2010-06-08-BrokenBBGun-06-large.jpg
http://themdg.org/pics/posts/2010-06-08-BrokenBBGun-07-large.jpg
http://themdg.org/pics/posts/2010-06-08-BrokenBBGun-09-large.jpg

Doc Sharptail · Member

Everything looks good- with the exception of the valve spring. It looks a little wonky/collapsed. I'd try giving it a stretch- just enough to get it straight at the top coils in your photo, and reassemble it.

Don't use that rem-oil in your air gun- 20 wt non-detergent oil is best. Rem oil, and other powder burner oils can be pretty hard on seals....

It doesn't take much dirt to prevent the valve stem from closing fully. A partially collapsed valve spring will do the same as dirt.

Regards,

Doc Sharptail

themdg · New Member

themdg · New Member

Success! After pulling on the spring bit, I lightly oiled most of the parts and reassembled. Went back together nicely. No air leaks, shoots great. Huzzah! Just in time for the weekend. My boy will be excited.

Thanks for your help, Doc. This has made me want to pull out another broken air gun. Smile

themdg · New Member

Yep...took it out today and put a few dozen bb's through it. Fires like a dream. Has such a nice solid thup as it shoots, in contrast to the almost-identical version I've had for 20 years or so. That one has more of a thudududud.

Doc Sharptail · Member

Glad you got it working again. I'm not sure on the house-hold 3in1 oil. 20 wt 3in1 for electric motors has done me in pinches, so to speak... simplest and best is the pint jug of air tool oil from wallyworld. It's cheap, long lasting, and zero additives.

You can maintain that repair a bit by keeping 2 or 3 pumps in the gun when it isn't being used. A bit of pressure will help keep the seals "up", and clean...
Regards,

Doc Sharptail

themdg · New Member