Not a lot of "guts" to this one, but there is an interesting use of slider pot for this dual-channel passive volume pedal. You can see from the photos that the mechanism is pretty simple, really. Move the foot treadle up and down and an L-shaped bracket moves a slider pot up and down. The middle picture shows you that the slider knob has a little gap cut into it, into which the moving bracket fits. Some grease in the area makes sure that the bracket does not catch on the slider knob.
And that's the key, really. You need to make sure the slider is firmly held and that whatever moves the slider up and down has enough play in it, and enough viscosity, to follow the slider knob as much as push it.
I didn't really have an urgent need for a dual volume pedal, but it was $15. I figured I could adapt it to some sort of filter pedal. There's already space and holes for 4 pots beyond the in and out jacks, and a sweet little battery compartment cast into the chassis. Seems promising.