I thought the position detector was the best part of the idea. :-)
Imagine a 1/4" tube 4 inches long. Brass maybe. The bottom 2 inches has a very fine coil winding 40 gauge, on a thin plastic insulating tube, several hundred turns. Inside this is a steel rod that acts as the pneumatic piston, 2 inches long. It has an aluminum rod screwed into it, just long enough to touch the skin surface. As someone mentioned before, the neg air pressure would bring the piston back so no springs needed.
The detector coil could provide a linear position output so a 0 to 1 inch movement would read the same as a 1 to 2 inch movement. might work.