Does brake fluid need to be changed periodically? I know engine oil, and often, transmission oil is drained and replaced. But I have not heard anyone changing brake fluid. In my car, I notice the brake fluid level is lower than it used to be.
Although car owner's manuals may not mention brake fluid replacement intervals, it is a good practice to completely drain out and replenish with fresh fluid every 80,000km or so.
Unlike engine or transmission oils, brake fluid does not flow around the brake components as a lubricant. Instead, its main function is to transmit hydraulic pressure from the master cylinder (directly linked to the brake pedal) to the individual callipers at each wheel.
The fluid therefore is not consumed. If you have noticed a drop in the reservoir level, this is because as the brake pads wear, more fluid is taken up by the piston chambers. When new and thicker pads are fitted, the pistons will be pushed back in their chambers, forcing fluid back into the reservoir.
Over time, brake fluid, like any chemical formulation, undergoes deterioration.
Replacing brake fluid is not as simple as draining it all out from a drain plug and then pouring in new fluid. The hydraulic network needs to be filled completely and any air bubbles must be evacuated.
The presence of even small amounts of air in the hydraulic lines causes the brake pedal to feel spongy and reduces braking performance.