You could just put some tape on the underside of the barrel where the pump arm meets the metal. That may give you enough friction to hold it in place.
If it's a Crosman 781, you could replace part 781A031 which is the spring and peg that holds the pump arm up. http://www.crosman.com/2004/pdf/manuals/crosman/781/C781-EVP.pdf
Answers & Comments
Go to www.crosman.com and they have a parts list that should help in getting the parts needed for replacement.
You could just put some tape on the underside of the barrel where the pump arm meets the metal. That may give you enough friction to hold it in place.
If it's a Crosman 781, you could replace part 781A031 which is the spring and peg that holds the pump arm up. http://www.crosman.com/2004/pdf/manuals/crosman/781/C781-EVP.pdf