I've had this gun for about 2 years now, and occasionally (accidentally) I have left it pumped / charged over night.  It is stored upright in my garage at all times.  After leaving it charged a few times, I have noticed that it will no longer hold air and pumping it requires about 100% more strength than it did previously.

It will hold a charge of 8 pumps for about 2 minutes, but no longer than that.  At the ten minute mark, it appears all air has bled off.

I have done the o-ring / rebuild kit on this rifle once already, and it resolved the issue for a very short while.  I'm thinking of doing it again, but wondering if anyone else has had a similar experience and if there is anything else I may be missing (other than not leaving it fully charged for hours on end).  Also concerned about the hard-pumping aspect of this situation - not sure if this is related to the air pressure bleed off or not.  Any advice is appreciated!

My best bet is, its your valve. A friend of mine had a 40yr old crosman pistol, (hos grandpas c130 or c140) anyway, it stopped holding air, we have to keep pumping it, and hear air pissing down the barrel. What happens is the valve gets oil, moisture, dirt, molten rubber(from the pump seal)  filth and grime begin to accumulate in there. They sell repair kits for like $4, includs the valve spring, the valve stem and the pump trap piece.
Its kind of hard to explain but, each pump, gets forced through a small hole on the face of your valve. That hole is sealed by a plastic piece called the check valve, and its pressed against the inside of the valvebody by the valve spring and the valve stem. Once you pull the trigger, the hammer whacks the back side ofthe valve and hits the valve stem, thereby allowing the air to rush past the stem and up through the transfer port, and eventually down your barrel.
So, either your check valve has gunk behind it, (or deformed from leaving it pumped for too long) or your valve stem seal has gunk under it/deformed. Only way that those valves fail. :)
PS, if you want more information, get a hold of timmy at mac1. Just google him, very well respected amongst the airgun community. He can get you the parts, or give you the part numbers to order via crosman, (part number may vary depending on what generation modle 392 you have.  or if youve got money burning a hole in your pocket, he can do a "steroid 392 tune" to get you a shit ton of power from that rifle. :)

I know benji made 3 generations of your gun, phase1 from eary on till 97, then phase2 from 97till 2007 andthe third phase from 2007 till present.

Here are 3rd pahse parts.
http://www.crosman.com/pdf/manuals/B397-EVP%20&%20PL4%20%20MFD%2010-07.pdf

Looks like youll need parts 9c, 9d, and 9e replaced, and maybe the valve orings as well.
Enjoy your classic benji dude! They stopped making them.