Igot a lesson on those 6400 brake boosters from Mr. Jim Carter himself ,had called his tech person about Bendix vacuum cylinder oil . He didn't know but said: Mr. Carter is in his office ask him about it! Mr. Carter said go buy hydraulic jack oil ...same thing! Give the booster 2 oz. shot of this oil twice a year ,like the service book says. My two boosters now work fine both are around 70 years old ! ! filled a 1.5 wheel cylinder cup with that oil set it out on the bench for a week . It never hurt it at all !