Ideally, the brake fluid reservoir
(typically mounted near the firewall on the driver's side) should always be at
or near the full mark. Brake fluid is part of a closed hydraulic system, and
under normal circumstances the level in the reservoir should not change
significantly.
As brake pads wear, though, the fluid
level may go down a little. That's a sign that you need new brake pads. A soft
or spongy pedal may also be caused by low brake fluid, which allows more air
into the brake lines. The fluid provides the hydraulic force that causes the
brake pads to be squeezed against discs or drums, stopping the car.
If the fluid level goes down frequently
or drops a lot in a short time, that means you have a leak somewhere in the
brake system, such as in the brake lines going to each wheel, the master
cylinder or in the hardware at one or more wheels. If that's the case, you need
to make an appointment with a repair shop.
Topping off the fluid reservoir every
week or two is only a stopgap measure, and further investigation by a mechanic
is required to find out why the fluid level keeps dropping.
