I have a 740 1990 the rotors seem hard to get off .After removing the pin locating the rotor as well as the calliper retaining bolts ,I had clear access tot he disc yet it didn't want to move .Do I need a rubber mallet with a block of wood to move the disc ? They sure seem tight ,I used wd40 to no avail .The car is free of corrosion so its got me puzzled .I just want to de glaze the discs with a rub down .I would be most grateful for any input ..