Brake Fluid Reservoir
The plastic container mounted on top of the master cylinder that holds brake fluid for the hydraulic system.
The brake fluid reservoir is a clear plastic container mounted directly on top of the master cylinder in the engine bay. Its primary purpose is to store a reserve of brake fluid, ensuring that the master cylinder always has access to the fluid needed to transmit hydraulic pressure to the brakes.
Because the reservoir is translucent, it allows drivers and technicians to visually inspect the brake fluid level and condition without opening the cap. This is important because brake fluid is hygroscopic (absorbs moisture from the air), and opening the cap unnecessarily exposes the fluid to atmospheric moisture, which degrades its boiling point.
The level of fluid in the reservoir naturally drops as the brake pads wear down, as more fluid is pushed into the calipers to compensate for the thinner pads. A sudden or severe drop in fluid level usually indicates a leak in the brake lines, hoses, calipers, or master cylinder itself.
The reservoir is sealed with a cap that often includes a rubber diaphragm that expands and contracts as the fluid level changes, keeping air and moisture sealed out of the hydraulic circuit.