Ron,
Back sinew is a lot easier to process then leg sinew: no pounding is needed. Here is a condensed version of the method I use:
I only use bison and elk back sinew, leave whatever meat is on it after processing the back straps, and then let it dry. After the sinew has dried (with meat attached) I break it down by working it over a round rod or dowel, much like shining your boots or shoes. I usually use the rod for tightening one of my vises and grab/hold both ends of the sinew with a pair of pliers. The dried meat will separate itself from the sinew during this stage. After the sinew is soft and pliable (usually a couple of minutes) pull the sinew through a metal comb/brush. My brush is a 1/2" piece of plywood with several dozen 3/4" x 1/4" staples shot through it and is the best thing I have found to separate the fibers. Total time it takes me to process, separate, and organize in magazine pages, enough bison/elk sinew for a 60" bow is less then a half an hour.
After using back sinew the first time I threw my leg sinew away, including all that had been processed.