A brake fluid leak is one of the few brake problems that can leave you with no brakes, so knowing the warning signs is genuinely important. Fluid loss is always a problem - there is no normal amount to leak. Here is how to catch one early.
A sinking or soft pedal
The most serious sign is a brake pedal that slowly sinks toward the floor under steady pressure, or suddenly feels soft and low. Losing fluid means losing hydraulic pressure - see when a soft pedal is urgent. A pedal that gets worse over days points strongly to a leak. Treat this as stop-driving urgent.
A dropping reservoir level
Check the brake fluid reservoir. A level that keeps falling, especially quickly, when the pads are not worn out means fluid is escaping somewhere - see why is my reservoir low. A slow drop can be normal pad wear; a fast drop or one with a wet trail is a leak.
Wet spots and clues at the wheels
Look for wet, oily spots on the inside of the wheels, on the ground where you park, or dampness along brake lines, hoses, and the master cylinder. Brake fluid is thin and clear-to-amber when fresh, darker when old - see brake fluid color meaning. A greasy film on a tire's inner edge often means a leaking caliper or wheel cylinder.
What to do
Any confirmed leak means do not drive until it is fixed - a burst line or hose can take out braking instantly; see brake hose replacement. Find and repair the source, bleed the system, refill with the correct fluid, and while you are there refresh worn pads and hardware.