* Air in the brake lines: This is a very common cause. Air compresses, while brake fluid doesn't, so air in the lines prevents proper pressure build-up. You'll need to bleed the brakes. This involves opening bleed valves on each caliper or wheel cylinder while someone pumps the brake pedal.
* Failed master cylinder: Even though the reservoir is full, the master cylinder itself could be internally damaged or failing to properly convert pedal pressure into hydraulic pressure. This is less likely if you recently topped it off and the level hasn't dropped.
* Failed brake calipers or wheel cylinders: These components are responsible for squeezing the brake pads against the rotors or drums. If they're seized, corroded, or internally leaking, you won't have brakes, even with full fluid. This often presents with one or more wheels not braking.
* Low brake fluid (despite full reservoir): A leak in the system somewhere (e.g., a leaking brake line, caliper, or wheel cylinder) could be causing a slow leak that's depleting fluid despite a seemingly full reservoir. Check all brake lines and components carefully for any signs of leaks (wet spots, fluid dripping).
* Brake booster failure: The brake booster assists the master cylinder, increasing the effectiveness of your braking. A faulty booster will result in very hard or no braking action.
* Broken or disconnected brake lines: A completely severed or disconnected line will prevent fluid from reaching the brakes.
What you should do:
1. Safety First: Do not drive the vehicle. It's extremely dangerous to operate a vehicle without brakes.
2. Visual Inspection: Carefully examine all brake lines, calipers, and wheel cylinders for leaks or damage.
3. Check the Brake Pedal: How does the pedal feel? Is it spongy, hard, or goes all the way to the floor? This will give some clues to the problem.
4. Bleeding the Brakes: If you're comfortable with it, try bleeding the brakes. If bleeding doesn't solve the problem, move on.
5. Professional Help: If you cannot find a leak or bleeding doesn't work, have a qualified mechanic diagnose and repair the problem immediately. Brake failure is a serious safety hazard.
Ignoring this issue is incredibly dangerous. Get your brakes checked by a professional as soon as possible.
