What Is the Brake Fluid Indicator


    – Brake Fluid Sight Gauge Characteristics

    – Brake fluid sight glass: recognize it and identify the cause of its ignition

The braking system is a crucial point in ensuring the safety of passengers and other road users. What role does the brake fluid sight glass play in the prevention of risks due to braking? Zoom in on everything you need to know about the brake fluid sight glass.

The role of the brake fluid sight glass

Contrary to popular belief, its function is to check the brake jar’s fluid level and not check the brake system’s pressure and efficiency.

Good to know: That’s why its ignition does not imply an emergency, as an engine oil sight glass would command, for example.

In an extreme case of leakage, the empty jar would cause air to intrude into the brake system with a loss of pressure and the brake system’s efficiency.

Please note: as the vehicle’s 4 wheels are powered two by two by different circuits, it is infrequent to have a total braking function loss.

Operation of the brake fluid sight glass

Brake fluid is stored in a jar attached to the master cylinder (brake pump controlled by the pedal). With two markings on its walls (mini and maxi), this jar is closed by a screwed cap. It contains the level sensor that controls the brake fluid sight glass. It has a float, which, in its lowest position, activates a contact that electrically feeds the brake fluid sight glass to the instrument cluster.

Brake fluid sight glass: recognizing it and identifying the cause of its activation

Identify the brake fluid sight glass

The brake fluid sight glass is often a red or yellow pictogram representing a circle surrounded by two arcs of a circle, associated with an exclamation mark. It should not be confused with two other sight glasses also related to the braking function:

    – The handbrake light (lit in the applied position) is identified by a very close or even identical symbol.

    – The brake pad wear indicator light also has a very similar design and is often yellow.

Good to know: for each vehicle, it is necessary to identify all these lights by consulting the maintenance booklet, which gives all the required indications.

Causes of brake fluid warning light illumination

The indicator light only comes when the brake fluid is at a low level in the jar (a few millimeters below the minimum level). There are several possible causes for this:

    – A brake fluid leak. In the short or long term, this leak can lead to losing part of the braking system and must be treated and diagnosed quickly.

    – An insufficient level of fluid but no leakage. It may be due to a lack of maintenance but does not pose an immediate danger to operation.

    – A malfunction of the level sensor. To check this, observe the level of liquid in the jar by looking through it. If it is correct, the sensor may be the cause.

Caution: As a general rule, it is recommended to check the motor fluid levels regularly. However, when checking the brake fluid level, keep in mind that the brake system comprises components that wear out (brake pads, brake linings). As the brakes are used, the friction material decreases, the discs and drums dig in, increasing the brake system’s volume. Therefore, the level of brake fluid drops typically in the jar, and it is useless to top it up. Indeed, when changing the brake pads, the fluid will automatically return to its maximum level if there is no leakage or anything.

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