Depending on how dirty, furry ( you should see mine - I could make a whole cat from the fur they've attracted ), gunked up they are taking the treadle plate off and cleaning the pivot holes and screws the lubing them might be a good idea. Also the ball socket on the bottom of the pitman arm ( metal one ) can need cleaned and lubed too.
The pivoting points on the big wheel should just need wiped out and oiled.
The belt should be just tight enough to not slip. Too tight can cause stiffness in the treadle, just like on an e-machine. Not to worry too much as the leather belts do stretch.
Joe