There is no way to get an infinite sustain when a sustain pedal is held and keys are released, as may be possible on other synthesizers. Released notes always decay, similar to on a piano with the sustain pedal down. To keep a note sustaining indefinitely, its key must be held down. This is a quirk of the CS-80 and something we decided to maintain on our GX-80.
You can read more about the Sustain section of the Left-Hand Controls in the GX-80 documentation: https://docs.cherryaudio.com/cherry-audio/instruments/gx-80/left