Brake fluid has the purpose of transferring the hydraulic force from the lever (master cyl) to the caliper. Any liquid will do this as liquids are not compressible. The enviorenment that the fluid operates in adds some complexity. Firstly is resistance to heat and in particular the boiling point of the pure liquid. Also the fluid has to look after the seals and keep them flexible etc. Brake fluid is the best liquid to do this. Works in all sorts of cars and bikes. Works well. DOT is a standard to regulate the performance of brake fluid. The numbers refer to the boiling point of the pure liquid and the liquid containing moisture.In general, the higher the number the higher the boiling temp. Most traditional brake fluid is polyethylene glycol based, and is hydroscopic. It absorbs water into solution wich then turns to steam at high temp and "boils" leading to loss of pressure transfer to the caliper and a soft level/pedal. The highest spec glycol fluid is DOT 5. DOT 5.1 was established as a standard for synthetic brake fluid based on silicon. DOT 5 and 5.1 are not compatible with each other and with the seals and should not be interchanged in a car motorbike or bicycle. The best thing to do is replace and top up the fluid with what the manufacturer designed the system for. Mineral oil will work, but the performance will not be what the manufacturer designed, especially in feel and performance especially at the limits, or high heat loads. Sometimes there is not viable sort cut. Buy the correct brake fluid for the designed application. Hope this helps.
