These bikes are all getting old & brake rotors are prone to cracking & are probably grooved or below the wear limits.
 
Stainless steel braking surface
Direct replacement
Sold individually
 
FITMENT:
BRAKEROTOR566 FITS - R65LS R75/6 R90/6 R90S R75/7 R80/7 R100/7 R100T, R100S R100RS R100RT FRONT & REAR
BRAKEROTOR005 FITS - 1979-1984 R65 1981-1987 R80G/S
BRAKEROTOR198 FITS - 1985-1987 R65 R80 R80RT
BRAKEROTOR067 FITS - 1992 R100 CLASSIC R100RS R100RT