Each jack requires a spring. Normally hog or boar bristle, which will last for centuries, by the way, I'm using high carbon monofilament (30 test) which I happen to have a lot of-so, why not use it?!
I use sewing needles of various diameters for my drills heated up and flattend and filed at the end, and then reheated and water hardened back. I use a Dremel-type felxible shaft grinder with a chuck that will hold these small homemade drills.
Once drilled thru the back and down through the toungue slot they receive a section of "bristle", which is later trimmed.
A jig was made to drill the axles for the tongue. The tongues will be angled back about 1-2 degrees to best facillitate the return when the key is disengaged. The drill is another needle slightly larger than the axle.
For the axle I use ordinary pins (the type that come with new men's dress shirts). The pins are a loose fit through the first tang and the tongue, but later tapped a few millimeters into the second tang of the jack. Here is a video of the process http://www.youtube.com/watch?v=pmrMr-TkND4
The extra bit is clipped off with small wire cutters and filed flat to the wood.