Diki, they are not buttons - take a look at the pictorial diagrams of the system and you'll see what I'm talking about. The triggering device is a series of PC strips that is fired by the velocity of the carbon impregnated rubber as it strikes the PC board beneath the keys. It's a different ballgame. I've had several apart over the years, and spent some time repairing them for friends.
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!