Brake fluid absorbs water over years, and water boils. What that does on a Caprock grade and how to prevent it.
Of all the maintenance items people skip, brake fluid is the one with the most direct safety consequence and the least attention.
Fluid absorbs water
Standard DOT 3 and DOT 4 brake fluid is hygroscopic — it deliberately absorbs moisture, so that water entering the system stays dispersed rather than pooling and corroding one spot. That is a sensible design and it has a cost.
Water boils at 212°F. Brake fluid is rated to boil far higher — but the more water it has absorbed, the lower its actual boiling point becomes. Fluid that started at 450°F dry can be down near 280°F after a few years of service.
What boiling does
Hydraulic systems work because liquid does not compress. Vapour does. When fluid boils in a hot caliper, you have compressible gas in the line — and pushing the pedal compresses the vapour rather than moving the pistons. The pedal goes long and soft and the brakes barely work.
This is fluid fade, and it is different from pad fade. Pad fade gives you a firm pedal that does not slow the vehicle. Fluid fade gives you a pedal that travels to the floor.
When to change it
Most manufacturers specify an interval measured in years rather than miles — commonly every two to three. Almost nobody follows it. It is an inexpensive service and it is the cheapest safety improvement available on most older vehicles.
Signs it is overdue
- Dark or murky fluid in the reservoir. Fresh fluid is clear to light amber.
- A pedal that feels softer than it used to.
- Any history of the pedal going long under heavy use.
- It has never been changed and the vehicle is more than a few years old.
Do not mix types
DOT 3, DOT 4 and DOT 5.1 are glycol-based and broadly compatible. DOT 5 is silicone-based and is not compatible with any of them. Mixing it into a glycol system causes real problems. Use what the reservoir cap specifies.
We do brake fluid service on site as part of a brake job or on its own. Call and we will come to the vehicle.
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