You have to find one that is intact to see what the inside is supposed to look like.

From the video, it looks like the spring broke at the bottom end or somehow came out of a retainer ring, because ot's not pulling down or pushing back.

Do you have a screw at the top of your machine that leads to the presser bar area? It could be it needs tightening; however, it sounds as if something broke inside the mechanism. Found this diagram and it may help you diagnose if you don't want to take it to a repair shop.

Hi Hob Lyon--Were you ever able to figure out what was wrong here? I am having the same issue with my presser lever (on the same machine) except my presser foot mechanism won't move at all. The spring is still connected on both ends but when I depress the lever its just loose and the foot won't lower.

Hello. I have the same Singer 57815c machine. I was having the same issue where the foot was slowly lowering. Then I noticed the bar had old sticky oil which I cleaned up and applied new oil (1-2 drop) to the bar inside the spring. It is now working fine. However the presser foot lifter is loose and still works.