Korg Concert C15 key sustain

A while ago I acquired a Korg C15 piano for cheap as I was looking for a full size weighted 88 key midi controller. It works fine but it seems like 3 of the keys in the middle of the keybed do not submit any sustain but still send on/off MIDI signals. I have opened it up to try and clean it and diagnose the problem but this thing is a beast, very heavy and when I got it apart I could seem to figure out a way to remove the keys to try and clean/replace the contacts… has anyone worked on this unit before amd could offer some insight? Thanks in advance!