Octave keypad failing or lost?
BluTak to the rescue.
Roll suitable sized ball of Blu Tak and plug in place of cork pad or on top of.
Take a tiny pice of light guage cigarette paper and place it firmly on top of the new Blu Tak pad.
A little talc or cue chalk helps also at this stage.
Press the new pad into the valve. It will form a perfect seal. The only thing to make sure of is that the surface remains dry and will open.
It's a great emergency solution, but having said that, I haven't used anything different in my alto for a year.