For 1979-1996 GMC P3500 Disc Brake Rotor Front Centric 1980 1981 1982 1983 1984. 1979 GMC P3500 Behind Hub – Disc Only; Position: Front;. 1980 GMC P3500 Behind Hub – Disc Only; Position: Front;. 1981 GMC P3500 Behind Hub – Disc Only; Position: Front;.