What are the signs of a faulty brake fluid reservoir?

Look out for visible cracks or leaks around the reservoir, a persistent drop in brake fluid level, and the brake warning light coming on. Leaking brake fluid is a safety concern and should be checked straight away.

What does the brake fluid reservoir do?

The brake fluid reservoir, or brake fluid tank, stores the hydraulic fluid that feeds the master cylinder and the rest of the braking system. It keeps the system topped up and allows you to check the fluid level at a glance.

Why is the fluid level dropping in my brake reservoir?

A falling level is often caused by worn brake pads, which is normal as the calliper pistons extend, or by a leak in the system, which is not. If the level keeps dropping after a top-up, have the brakes inspected for a leak.

How much does a brake fluid reservoir cost?

The price of a brake fluid reservoir depends on your exact vehicle and the brand you choose. Enter your registration above to see parts confirmed to fit, then compare prices from the brands we stock on this page. Free UK delivery on orders over £50.

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Brake Fluid Reservoir buying guide — what to know before you buy

The brake fluid reservoir stores the hydraulic fluid used by the braking system and keeps the system topped up as brake pads wear and calliper pistons extend further. A cracked or leaking reservoir causes brake fluid loss that leads to brake failure — the most dangerous possible braking system fault.

Brake fluid reservoir function

The reservoir is connected to the brake master cylinder and maintains a supply of brake fluid under a diaphragm or float-fitted cap. As brake pads wear down the calliper pistons extend, displacing fluid from the callipers back into the reservoir to compensate. The integral level sensor triggers the brake warning light when the level drops below a threshold. The cap vent allows air in to replace the fluid volume as the system uses it, while preventing moisture ingress.

The clutch master cylinder reservoir shares the same tank on many vehicles with a hydraulic clutch, meaning a leak can affect both braking and clutch systems simultaneously.

Causes of reservoir failure

Reservoirs are made from translucent plastic to allow visual level inspection. Over time UV exposure, heat cycling, and contact with brake fluid cause the plastic to become brittle and crack, particularly around the level sensor boss or the outlet fittings. Physical damage from overfilling, fitting the wrong cap, or an impact to the reservoir are also causes of failure.

If the brake warning light illuminates, check the reservoir level immediately. A sudden drop in level indicates a leak somewhere in the system, not just a worn reservoir — inspect all brake hoses, callipers, and line connections before assuming the reservoir itself is the source.

Brake warning light — check level immediately
Brake fluid smell or visible leak under bonnet
Cracked or brittle plastic reservoir body
Level sensor malfunction causing false warning
Cap vent blocked causing pressure build-up

Replacement

Brake fluid reservoir replacement requires draining the reservoir, disconnecting the hose to the master cylinder, and transferring the level sensor if it is a separate item. Always use fresh brake fluid of the correct specification (DOT 4 for most UK vehicles; check the cap for the specification) and bleed the system after any reservoir disturbance. Use the registration lookup to confirm the correct reservoir for your vehicle.