I did a search and couldn't find the post where someone mentioned that they had cured their belt sander from breaking old belts by reducing the spring pressure on the front roller. I am currently working on a set of furniture and bookshelves for our family room/office and had some belts on hand that were at least 2 years old. The first one broke in a matter of seconds. Then I recalled reading the tip about reducing the spring pressure. I looked at my sander and saw that it would be possible to place a shim between the spring release and the body of the sander. I added a 1/16" thick wood shim with some double sided tape on one side to may sander. You can see the shim in the photo
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The result was that I have yet to break the second belt from the same two year+ batch of belts, despite using the sander for nearly 20 minutes so far.
My sincerest thanks to whoever posted the idea to reduce the spring pressure with a shim.