Unless you are very handy with tools I recommend one of three things.
Crosman still has parts available for this rifle and they can be ordered through Crosman. But this will require you to dissemble your rifle to locate the bad o-ring or seal. You will have to download the parts list ( See the link below) there are 10 variations of the 760 so you will have to identify which one is yours.
Crosman also offers a service to locate repair shops that repair older air guns or rifles. ( see the link below ) when it opens, click on "Find a service center."
Your last choice is to just purchase a new one for $30 ( see the link below ) For the price, this would be my choice.
See link Below! There are about 11 different versions of the 760, See the link at the bottom of this page to select the one you need. (It's all the Crosman air rifle models.)
Manufactures do not publish the distance that air guns fire they just say several hundred yards to be safe.
PSI: It depends on how many times you pump the forearm. But Crosman says to never pump it more than 10 times. The 760 is rated at 625 FPS with a BB and 600 FPS with a pellet.
First of all the 760 Pumpmaster can shoot any brand of BB's without any harm. But if you want Copperhead then I suggest you look at any sporting goods store. Like Walmart or Kmart. If you can not locate them then I suggest looking Online at Pyramydair they are the largest air gun store online.
check the link below and contact customer service. Crosman list that some parts are still available for this rifle.
No- and it would be very dangerous to try. Pellet guns and airsoft are two very different things.
Crosman has most all of their air rifles Owners manuals and parts manuals online. See the link below and download you parts manual. This will give you a start.
The model 760B has a scope rail built in. I am not sure about the older 760's, there were 9 older versions made. I would not go crazy and purchase an expensive scope for it considering the rifle only cost around $30, but yes it is scopeable. Crosman make a scope that fits the 760 and is not expensive. see the link below
There are 10 different versions of the air rifle. Here is the drawings for the first one ( see Link Below) go to the Crosman home page and click on "download a manual" to see the other 9 versions.
Go to (The link below ) and find your parts and owners manual for your gun or rifle. Crosman keeps a web page for older airguns. They also have a list of service centers
I assume you are asking about the 760 pumpmaster XLS. Their are 9 different variations of the 760. The Crosman 760 is a BB repeater or single shot 177-pellet rifle. But your question is how do you shoot it? Load the rifle with either a Pellet or BB. Pump it several times NOT more that 10 times. Usually 8 pumps is more than enough. Aim at your target grip the rifle fairly tight in your right hand but do not grip it tight in your left hand (Hold it easily with your left hand.) Air rifles have a different kick than regular gunpowder rifles. Holding it tightly in both hands will cause you to pull off target. Relax and take a deep breath and breathe out slowly. Aim at your target using the sights. Squeeze the trigger. Do not yank the trigger or it will pull you off target. Once the rifle fires keep you sights on the target. (Basically follow through with your shot) adjust your next shot to compensate for any variations that you made on your last shot. The 760 is a smoothbore barrel rifle (most all BB rifles are smooth bore) so it is not as accurate as a rifled bore Air Rifle. ) However with practice you will get better at knowing you rifle, so keep practicing.
The Crosman 760 Powermaster was made from 1964 until 1991 in about eight different variation's. Today it called the 760 Pumpmaster. The rifle was never very expensive and a brand new collectors edition cost $85, but most new 760 will cost around $60. To find an old one I would try Craigslist or gunsamerica. The version you want is the 5th edition 1977-80 that had manual cocking. Good luck.