As recommended, review the sawdust session, especially at the end where Nick shows how to wind the tension spring. I just finished rebuilding a 1960 Magna belt sander that had the same issue. I was convinced it was a basket case but after reviewing the video (more than once) I was able to disassemble, clean, lubricate, reassemble and set the tension to get it working properly. Now it holds tension right and tracks just fine once set. I did find out that I was turning the tension adjusting knob the wrong way at first. Also if you don't have a belt sander owners manual and parts list, you should order them from SS. They are very inexpensive and useful.
1954 Mk 5 SN 269454, 1955 Mk 5 SN 316013, 1960 Mk 5 SN 360792, 1962 Mk 5 SN 380102, Magna band saw, (2) jointers, (1) belt sander, (1) air compressor, (1) jig saw, (1) strip sander, (1) 20" scroll saw, DC 3300 dust collector, Sawsmith RAS, Craftsman table saw, 13" DeWalt planer, Triton 3 1/4 HP plunge router & table