You shouldn't be able to break sinew easily even using a dowel. If you're strong or really pull really hard you may occasional break it but you should not be able to do easily.

I once received an order of sinew quite a while ago that was made with the wrong thread and it broke quite easy. I only sold a 7 or 8 rolls before it was brought to my attention and I though I traced down and replaced all the defective rolls. How long ago did you get this from me?

Do you have any left? If so I would love to see it!