A single master cylinder was stock for a 4 wheel drum system and will work. If you went to a different system like disc/drum a dual master cylinder is needed. If you went to 4 wheel discs, in theory a single could work. The reason to add front discs is to make it safer to drive, not more dangerous. edit: A good friend of mine was driving his 64 Galaxy conv. when his single master failed and he got good and scrayed going through a intersection inexpertly. It was converted to a dual system after that and this was from a very OEM numbers matching type of guy.
