Interesting thought there Gary, but I have never heard that brake fluid had to reach a specific temperature to work properly, at least not in real cars. For a brake system like sim racing, it might be the case but I have no clue on that.
The reason brake fluid is used in cars is because it was designed to have no compressible properties like water or other fluids have, and the brake fluid does not expand under heat like other fluids do. These properties remain the same regardless of temperature. Put simply, when you step on your brake pedal with a properly setup brake system with no air in the lines, it will be like stepping on a rod connected directly to the brakes. Vegetable oil will be very similar, though there are things about vegetable oil that make it less than desirable such as expansion under heat, heheh.