The maintenance manual of our 59 Montclair named the brake fluid as “MK-19542”. What does that mean nowadays? DOT-3? DOT-4? There is no mark on the main brake cylinder, I guess it is a replacement part. Has anyone an idea what kind of fluid we should take?
Typically the higher the dot number the higher the boiling point of the brake fluid. I would think you could use either dot3 or dot4 in your ’59 Montclair without any problems. Remember that in 1959 brake fluid was brake fluid. If it said “brake fluid” on the can you could use it in any car. Drum brakes do not have the high temps generated by disc brakes. Many of us, though, are switching to dot5 brake fluid. I am not sure of its availibity in Germany at this point. It is hydrophobic (it does not attract moisture) and thus does not have the issues with moisture in the brake system that the lower dot numbers have. It has viscosity (thickness) stability to a higher temp than dot3 and dot4. However, you ABSOLUTELY MUST NOT mix dot5 with dot3 or dot4. To use dot5 you must flush the brake system completely (something you should consider occassionally anyway) and then fill with dot5. Hope this helps a little.
The problem with DOT 5 fluid is that although it does not absorb moisture, if you use it in a vented system, such as those before disc brakes, the moisture that does enter, and moisture will enter, will collect in the low points of the system, like low bends in the lines, This results in rust in those places, and also, if your car is subject to freezing temps, frozen blocks in the line.