I have the same issue with my PX-130. It cost $1.5 a piece and I have to wait for a month until it arrived at Casio service center, then they will send it to me. A month is too long for me, my daughter used to use it as a practice for her Piano Course.
So, I have to do some DIY creativity. The weight of this rubber cap is less than 1 gram. Its functionality is as a dumper and maintains the height of hammer tuts. So, I try to resemble it. I cut an aquarium hose(made from rubber) with the same width of the hummer cap, I stretch the hole wide enough. Then I cut a tiny slice of rubber damper of outdoor air conditioning machine with the same tick of top part of the hammer cap. I put that tiny slice to the hose and put them at the original place of the hammer cap. Rubber hose will keep the tiny slice in its place. You can put a little bit of strong glue to make it placed by sure(without moving easily).
Before I do that, I put off the hammer cap from rightest of tuts(rare to use in playing piano) and put it into the broken hammer cap. So actually I change the rightest tut, not the broken one.
After all of that, my PX-130 back to normal again without any issue of feel when the tut is pressed. Hopely my experience could give some idea for you.
