G'day! I picked up a FV-50L for cheap the other day and tried it for the first time today.
I've found that the first 40% of the travel doesn't do much then the volume all comes in a rush in the last half of the travel. It's really finicky to set a medium volume level and not have it change when I lift my foot off the pedal.
I've haven't experienced this pedal, but I'll add something that may be relevant.
When I did up a guitar, I added a Linear Pot for the tone instead of a Logorythmic. The result was that the tone didn't change until I had turned the pot 40-50%. If this pedal has been diddled with, it may have had a Linear pot installed instead of a Logorythmic. Or as it's a volume pedal, it may be vice-versa....
The FV-50L is designed for low impedance signals, and will only work properly after a buffered pedal (such as a Boss or similar). If you plug the guitar straight into it, it won't work right.