If your vehicle is showing clear or yellowish fluid leaking under the car near the wheels or firewall, here are the most likely causes and the parts you'll need to fix each one.
Brake fluid is typically clear to yellowish and feels oily. A failing seal in the brake master cylinder will cause this fluid to leak directly down the firewall.
The internal piston seals in the brake calipers can degrade over time, allowing clear/yellowish brake fluid to leak down the inside of the wheels.
Rubber brake hoses flex constantly and can crack. A very common installation error is twisting the hose when installing a new caliper, which stresses the rubber and causes leaks near the wheel.
If brake work or a fluid flush was recently performed, a bleeder screw that was not fully tightened will allow brake fluid to seep out and drip down the wheel.
On manual transmission vehicles, the clutch hydraulic system uses brake fluid. A failing clutch master cylinder will leak clear/yellowish fluid down the firewall.