Now joining the conversation. Saw a solution similar to Herruez's on YouTube ((54) How to Fix hammer Digital EP casio previa series - YouTube).
My PX-400R has hardly been used but the caps have almost all turned to mush. Looking at the key and hammer designs, I am thinking of just taking off all caps without replacement, even if they are solid. I will then stick a flat pad at the top of the slot (approx .8 cm wide) in the key where the hammer (tuts?) fits. I can get back the key height that way. I haven't figured out the required thickness as yet - but I think that there is some tolerance there as that strip to the back of the keys presses down on them if they are raised too high. The other problem I see is wear and tear. This can be reduced with some of that white lubricant that's all over the structure and by using a plastic or hard rubber pad. There is the possibility of using shrink wrap tubing around the hammer as well. Will advise.
