The mysteriously sinking brake pedal

Question: I have a 2001 Chevrolet Suburban 1500 that all of a sudden lost brake power. I came to a stop sign, and the brake pedal went halfway down. I was able to pump the brakes and build up pressure, but at subsequent stops, the pedal went to the floor.

There was a smell of burned brake fluid, but no fluid was seen under the hood or on the ground after the car was parked.

I had it towed to the shop the next day. The brakes were tested and noted to be fine. The reservoir was full and did not require any additional brake fluid.

What could cause this?

Answer: I'm at a loss to explain the odor you mentioned, but the sinking pedal rings clear.

In a follow-up message, you indicated the brakes were not hot. The most likely cause of a sinking pedal with no external leakage is a faulty brake master cylinder that's leaking internally. Were the brakes hot, we might consider boiling fluid due to moisture contamination or friction material gassing.

Try pressing and holding the pedal (one time, continuously, engine not running - the hard pedal is easier to evaluate) and see if the pedal slowly sinks to the floor. Repeat as needed, pressing softer or harder, and perhaps with the engine running to tempt a symptom recurrence.

If the pedal sinks, this confirms a faulty master cylinder. Another possibility is that crud at the bottom of the fluid reservoir is preventing consistent take-up of fluid by the cylinder.

Perhaps the odor was the result of a minor oil or coolant leak? Fortunately, the master cylinder is a fairly inexpensive and easily renewed component.

Q: My 2002 Mustang has begun to do some strange things when I start it. It will growl like it doesn't want to start, and the radio will begin playing loudly and the clock resets to noon. What could cause this electrical problem?

A: It sounds as if your battery may be failing or is becoming discharged because of either a charging-system fault or a parasitic drain.

The radio and clock are resetting due to low system voltage as you crank the engine.

Dirty battery terminals are another possibility that could cause these symptoms.

Testing a battery can be done in two ways. One is a load test. The fully charged battery is subjected to a very strong load for 15 seconds, checking to see if it holds acceptable voltage.

A more techie test is cell conductance, which has become the more common method due to high accuracy. Whether the battery passes muster or not, it's still a good idea to check for proper charging-system performance and the possibility of a key-off parasitic drain.

Examples of such a drain are the glove box or trunk light remaining on or a body control module failing to fall asleep.

An easy test for dirty battery terminals is voltage drop across each cable clamp and its battery post as the engine is cranked. More than a tiny difference between the two indicates a faulty connection.

These tests are all quick to perform and ensure that a single trip to the shop will be all that's needed.