Use only the brake fluid with the DOT specification recommended in your owner’s manual. There are three main specifications: DOT 3, DOT 4 and DOT 5, each having its own properties. It is possible to use DOT 4 brake fluid in some cars that call for DOT 3 fluid, but never the reverse, and DOT 5 brake fluid can be used only in cars that call for that specification. Note that most cars indicate the required brake fluid DOT at the reservoir cap. This is also a good time to check how well the reservoir cap seals.
If the brake fluid level is significantly below the “Min” or “Add” line, you should have your brakes inspected for excessive wear. As brake pads wear down, brake fluid can channel itself out of the brake lines into the calipers. [6] X Research source It is also possible to have the brake fluid reservoir full and not have the brake fluid reaching the master cylinder. If your brakes feel spongy even with the reservoir full, take your car to a mechanic.
Your mechanic should have both an optical refractometer and electronic brake fluid tester to perform these tests for you as part of an automotive checkup.
