James is right about pushing the trigger forward to set it other wise it will work as a regular trigger. I highly doubt that you could file/hone the catch in the hammer enough to move the sear enough to have the knock off stop the sear from catching.
To me it sounds like you have a hammer fly problem. Do you have any pictures of the hammer so we can see it? Also will the hammer catch in the half cock notch?
There are a couple places to get parts as soon as you know the problem.