A car runs on six fluids, and each fails differently. Engine oil degrades by heat and contamination, coolant loses its corrosion inhibitors, brake fluid absorbs moisture from the air, and transmission fluid provides a specific friction characteristic that the wrong product will not reproduce.
Why does brake fluid go bad even if I rarely drive?
Brake fluid is hygroscopic, drawing moisture from the atmosphere through hoses and sealseven on a car that is barely driven. Water lowers its boiling point substantially, so underheavy braking the fluid near the caliper can flash to vapor, and vapor compresses in a wayliquid does not. The result is a soft pedal at exactly the moment maximum braking is needed,on a system whose pads and rotors may be perfect.
Moisture also corrodes the system internally, attacking caliper bores and the fine passagesinside the anti lock brake module, which is by far the most expensive component in a brakingsystem to replace. Fluid condition is measurable in minutes rather than a matter of opinion.
Understanding Car Fluids: Questions
Why does understanding car fluids matter?
Understanding car fluids matters because each fails differently: oil degrades with heat, coolant loses its corrosion protection, brake fluid absorbs water, and transmission fluid must match the specification.
How often should brake fluid be changed?
Most manufacturers specify every two to three years, because brake fluid absorbs moisture from the air over time.
Can I use any transmission fluid?
No. Use the exact specification in your owner's manual; the wrong fluid can change how the transmission shifts.

