A local storage locker buyout reseller had a 31-15 laying in a corner for a year. I bought it the other day for 10 dollars. Having a project with 4/5 oz leather and 138 thread I thought it would do the job. I got it adjusted for the 138 thread and found a few issues.
#1. The tension release lever is broken. It sews ok but the stitch isnt to my liking as far as locking in the center of the material goes.
#2. When set to max stitch length there is a slight binding/clunk as the feed dogs drop at first it would also bird nest when set to max length. I adjusted the feed dog centering according to the navy manual. The clunk is mostly gone and the birdnest issue went away.
#3. While not a show stoppper this may be telling, most of the screws on this machine have been well abused by screwdrivers.
So my thoughts are..
Buy the parts to fix release and replace the tension assy possibly the hook and race at the same time. Not sure if a new feedbar would fix the clunk issue.
Or walk away and find something else that will do flat felled seams in 5oz leather chaps.