If your vehicle is showing brake fluid reservoir needs frequent refilling, here are the most likely causes and the parts you'll need to fix each one.
As brake pads wear down, the caliper pistons extend further to compensate, causing the fluid level in the reservoir to naturally drop. Frequently topping off the fluid without checking the pads can mask dangerously worn brakes.
The rubber piston seals inside the brake calipers degrade over time due to heat and moisture, causing brake fluid to leak out around the pistons and onto the wheels or ground.
Steel brake lines are highly susceptible to rust, and flexible rubber brake hoses can crack or chafe over time, leading to pinhole leaks that quickly drain the hydraulic system.
If brake work was recently performed, a bleeder screw or the banjo bolt connecting the brake hose to the caliper may not have been torqued properly, or the copper crush washers were reused, causing a steady drip.
The internal seals of the master cylinder can fail. This can cause an external leak down the engine firewall, or a hidden internal leak where fluid is sucked directly into the brake booster.
In many manual transmission vehicles, the clutch hydraulic system shares the fluid reservoir with the brakes. A leak in the clutch master or slave cylinder will drain the shared reservoir.