On this particular keyboard keys like Return and 0 on the keypad were almost non-operative. But after the cleaning, the whole keyboard is working like new! :) I believe that the most important thing to clean is the black graphite that comes in contact with the gold fingers on the keyboard. On the most used keys I noticed it was shiny and had a type of imprint like the gold fingers it would be constantly coming in contact with. I used the eraser on these till the black graphite wasn't shiny anymore and the imprint was no longer visible.
Unfortunately this same procedure cannot be done on that PET 2001 with business keyboard as the "switch" for each key is inside each key. I may be able to bring these metal fingers back to life by spraying them with DeoxIT. Maybe I could transplant the metal fingers from another keyboard, but I am not sure if the metal fingers are attached to the plastic in any sort of way.