Search for the Seizure

Finding out what's responsible for a seized brake caliper

Tool Briefing offers real-world tips and advice for using specific tools to accomplish specific tasks. The Customer Concern is shown below, along with a brief list of factory bulletins addressing the same problem. Remember, the tool information might apply to many other vehicles too.

Vehicle Application:

• 2004 Mazda 6, 3.0L engine

Customer Concern:

Came in with the brakes locking up and smoking brake pads. The left front caliper was frozen and had a collapsed brake hose. After replacing those parts, the same problem returned.

Avg. Reported Mileage:

• 63,000 miles



Separate the master cylinder from the brake booster and determine if the master cylinder piston may be hanging up and not returning all the way. If so, the master cylinder is faulty. This usually happens as the result of brake fluid contamination.

Tech Tips

These models are known to suffer brake fluid contamination. If brake fluid has turned green, flush the system to prevent further complications.


• Brake pressure test gauge/test kit

• Brake fluid pressure bleeder/flush tool

• Brake fluid testers

Contaminated brake fluid

Brake fluid contamination is more common than most techs or their customers realize. Brake fluid contains alcohol, and it absorbs moisture even in a sealed system. Seams in the steel brake line are brazed with a copper alloy, and as moisture content in the brake fluid increases, the copper corrodes and dissolves in the fluid. Over time it attacks the rubber seals, and tiny passages may become clogged as the rubber deteriorates. This is why many car manufacturers recommend flushing the brake fluid semi-annually.

Two different tools are used to check for contamination: test strips, and an electronic conductivity tester. Test strips are chemically treated slips of absorbent paper that turn different colors in the presence of copper. Brake fluid naturally darkens over time in the system, but that tells you little about its condition. The chemically treated test strips are far more reliable. They’re also an inexpensive and convincing sales tool. An electronic conductivity tester costs a bit more, but it’s a sales tool that can help avoid brake fluid contamination. Instead of detecting dissolved copper, it measures the moisture content in the brake fluid. If the fluid is tested regularly and changed when moisture is above acceptable limits, the problem experienced in this Mazda might have been avoided. Using a conductivity tester is easy: just dip the probe into the fluid reservoir and read the meter.

An optical refractometer will also show the water content in brake fluid. It’s inexpensive, simple to use and perfectly reliable. Since only two drops of fluid are needed for testing, a refractometer can be used to sample brake fluid from both the reservoir and the caliper/wheel cylinder.

Brake fluid flushing

There are several different tools for flushing a hydraulic brake system. On most Anti-lock Brake Systems (ABS), the valves must be commanded open to flush all the old fluid from the valve body. A bi-directional scan tool that communicates with the ABS control unit will do the job. However, even if you can’t open the ABS valves, a fluid flush will still change over 95 percent of the fluid in the system.

Pressure bleeders and brake flush machines work by forcing clean brake fluid through the system from the fluid reservoir all the way down through each open bleeder. Some use an electric pump, some use shop air, but either way the pressure must be regulated to avoid damaging seals and valves. Typically, about 30 psi is enough, although higher pressure is required on some hybrid vehicles. The job usually takes about ten minutes, but with set-up time it’s easy to justify charging an hour of labor.

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