The print hammer rubber cap on my Teletype ASR 33 had some cavities in it when I got it but apart from that, it was in good shape and had not become soft. I purchased a tube of LiquiSole, a rubber like compound which I used to fill the cavities in the rubber cap with and left to harden for 24 hours. I then put on another layer to make the hammer head cap nice and even. After hardening for 48 hours I tested the system which now prints even and clear characters.
