I recently got my old Montgomery Ward jigsaw working, including the sawdust blower. This is a little air pump with an outlet to attach to a small hose, directed at the saw cut, to clear the cut line of sawdust.
The blower is attached to the upper blade holder return spring. The blower linkage is attached to this spring by a nut that holds two small clips; unscrew this nut to detach it from the spring. Remove the clips and the nut on the bottom. You can now remove the two screws that hold the blower to the saw frame.
Pull the plunger (attached to the U-shaped linkage) out of the blower body. The plunger is a simple piston consisting of a leather disk sandwiched between two brass disks on the plunger shaft. Dig the old leather disk out (you might have to slice it up to get it out).
Make a new disk out of leather. I first traced the outline of the brass plunger disk on the leather blank, and pressed the plunger into the leather to mark the center attachment point. Drill a hole at this mark, about 1/8" in diameter. I used a sharp wood chisel to punch out the disk, starting with a hexagon shape, then trimming the points to form a circle. Don't worry about making a perfect circle; the leather is soft enough to squish into the correct shape.
When the disk is the right size (mine ended up about 1/32" outside the pencil line), make a cut from the hole to the edge of the disk. Spread the cut edges apart to squeeze the disk in between the brass disks. (Since my leather was a bit thicker than the original piece, I ended up squishing the disk in the jaws of a vise to make it thinner.) Work the disk into the plunger. When it's evenly sandwiched between the brass disks, fit it to the blower body to make sure it fits correctly. It should be a snug fit, not too tight nor too loose. Lubricate the disk with a little oil.
I ended up making 3 disks before I got one that fit. Judging from the original one, a properly-fitted replacement plunger leather should last a long time.