Brake Fluid Boiling Point Calculator
Brake fluid pulls moisture out of the air and its boiling point falls as it does. Estimate where yours is now from its age and your climate.
Your brake fluid
- An estimate from age and climate. A test strip or an electronic tester measures your actual fluid.
- Moisture collects at the lowest points in the system, which are the calipers, so the fluid there is worse than the reservoir suggests.
- Brake fluid absorbs water through flexible hoses as well as the cap. A sealed reservoir does not exempt it.
This is an estimate, not a test. Boiling brake fluid causes total pedal loss. If the vehicle sees track use, towing or mountain descents, test the fluid rather than relying on a calculation, and flush it on schedule regardless.
Why brake fluid goes off
Glycol brake fluid is hygroscopic, meaning it pulls water out of the air and holds it. That is deliberate. Water in the system would otherwise pool and corrode from the inside, so the fluid absorbs and disperses it.
The price is the boiling point. Fresh DOT 4 boils at 446F. At the federal wet test point of 3.7% water it boils at 311F, a loss of 135 degrees, and that happens in two to three years of ordinary driving.
What boiling actually does
Hydraulics work because liquid does not compress. Boil the fluid and you get vapour in the lines, which compresses beautifully. Push the pedal and you compress bubbles instead of squeezing pads.
It shows up exactly when you cannot afford it. The fluid boils after sustained heavy braking, so the pedal goes long or hits the floor part way down a mountain pass or at the end of a track session. Let it cool and it comes back, which is why the fault often goes undiagnosed.
Change it on time
Two years is the normal interval, once a year for anything that sees hard use. It is a cheap job that almost nobody does, because unlike pads there is nothing to see and no noise to hear.
Frequently asked questions
How often should brake fluid be changed?
Every two years for normal road use, and annually for track days, towing or mountain driving. Some manufacturers list longer intervals, but moisture absorption does not care what the manual says, and the test is cheap.
Can I mix DOT 3 and DOT 4?
Yes, they are both glycol based and fully compatible, and topping up DOT 3 with DOT 4 raises the boiling point slightly. Never mix either with DOT 5, which is silicone and does not mix at all. DOT 5.1 is glycol despite the name and is compatible with 3 and 4.
How do I know if my brake fluid is bad?
Colour is a weak indicator, since fluid darkens from heat and age whether or not it has absorbed water. Test strips or an electronic tester measure moisture directly for a few dollars. Age is the most reliable guide short of testing.
What is DOT 5 and should I use it?
Silicone fluid that does not absorb water at all. That sounds ideal but means any water that does get in stays as free water and corrodes in one spot. It is also more compressible when hot. Do not use it in a system designed for glycol, and do not use it with ABS.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.