I have an old one that I haven't yet cleaned up and resealed, and that channel for the bbs that runs along the top of the two receiver halves is caked with all kinds of crud and deposits. I'd split the receiver and take a look there. Also, do the two receiver halves mate well, and are the screws tight?
There's nothing better than a simple fix that doesn't cost anything, glad you got it working. You've inspired me to get working on that old, neglected 760 that's been sitting on a shelf in my garage.
It's a self-cocker, and I'm pretty sure it's a second variant as well. It has the checkered plastic stock and the checkered plastic pump handle. The pump handle is busted in half though, and I haven't been able to find another checkered one. I'm probably going to have to settle for a plain one. It doesn't seem to have had much use. Currently, it won't pump enough air to cock it, even though the pump cup looks nearly new. I might try the transmission "stop leak" on it just to see if it will pump up enough to cock it. If not, I'll replace the pump cup and see if it needs to be resealed. Ultimately, if it seems to be a good shooter, I'll put a rifled barrel on it and only fire pellets through it. I have three more (various) of these and one 761XL.