Dirt, grime and dried oil can interfere with a lot of functions on the machine. Keep up a cleaning and oiling routinge and the machine will eventually return to former glory.
There are help to be found. Almost half way down
this page there is a guide on how to clean and sort out the old type tensioner on a 15. I think it shold be the same as on your 115. Tension release is connected to the presser bar lifter, and you have to make sure all the parts behind the faceplate is clean and oiled. It can take a bit of effort and a few turns to get it nice again, pay particularly attention to the link from presser bar lifter to the tension release parts. I assume it pushes against the release pin from behind there.
If somone has an old type 15 tensioner, they might measure the release pin. I would not start cutting it just yet, at least not until everything is clean, oiled and sorted out. The reassembly parts here are very dirty, they should go back together clean and dry.
