Restoring rubber keyboard on a logic analyzer

Gentlepeople, I have a Philips logic analyzer (PM3585) which is about 20 years old at this point. It seems to be basically functional except for the keyboard, which unfortunately is a critical part. This is one of those molded rubber type, with a circuit board behind the rubber that has contact areas made of carbon film (at least they are black in color) and on the back of each key a small cylindrical bump also coated with carbon. Some of the buttons work but most don't seem to even if I press hard. I've disassembled the keyboard, which was easy enough. Inspection shows no damage and no signs of corrosion or contamination. I wiped everything with isopropyl alcohol anyway. The result is no change in behavior. Any suggestions for what to do next? paul