Under the Hood: Troubleshooting a wonky brake warning light

McClatchy-Tribune Information Services

July 29, 2019

Q: The red brake warning light on my 2005 Ford F-150 comes on at various times. Can you suggest what I might do to check for the cause?

ó Ted

A: According to my ALLDATA wiring diagrams, your F-150 has three possible or intended causes for the red brake indicator to illuminate.

First is the bulb check process when you roll the key to the crank position as you start the engine. This proves the light bulb works.

Once the truck is running, the other two possible causes are the parking brake switch and the master cylinder reservoir low fluid level switch. A grounded wire leading to either switch is a distant fourth possibility.

Letís start by checking the master cylinder fluid level. If it is low, filling it should do the trick to extinguish the light. Use only fresh DOT 3 brake fluid from a sealed container. A low reservoir is cause for further investigation. It would be prudent to check brake lines, hoses, calipers and master cylinder for leakage. Itís also possible the fluid level is low due to extended caliper piston positions as a result of worn or thin brake pads. This is the more likely situation.

If the fluid level looks OK, weíll next look for a faulty low fluid warning switch. It sounds like the light is intermittent so youíll need to wait until the next time it comes on to do the following: With key on and engine off, unplug the master cylinder fluid reservoirís 3-terminal connector. If the light goes out, there is something wrong with the fluid level float switch.

The parking brake switch is next. Itís a bit more difficult to get to, as it is located on the foot pedal mechanism. Try this first: Pull back and hold the brake release handle, and slowly press the pedal up and down a couple times near the top of travel, observing the warning light like a hawk. If it remains on at the exact same intensity, the parking brake switch warrants further inspection. If the light changes in intensity even slightly as the pedal is operated, the parking brake switch is probably not the cause (light is already on due to another cause, becomes brighter due to two ground paths).

Next try pulling the pedal back slightly beyond its top-stop with the top of your toes while pulling on the release. If this cancels the warning light, the switch requires adjustment or repair.

Now it may be time to get a little dirty and torque your back! Unplugging the single wire leading to the parking brake switch will cancel the light if it is the offending part. Sometimes these primitive switches come loose or fall apart, or if the wire leading to it becomes snarled in the ratchet mechanism, the resulting grounded circuit will turn on the light, likely intermittently.

Q: I was just told when having my tires rotated on my Yukon 4x4 that my right front wheel bearing was loose. Being an old timer, Iíve done wheel bearings before. I was surprised to see there isnít a dust cap as Iíve had in the past. What gives?

ó Carl G.

A: Your Yukon, like other GM light trucks, utilizes an integrated hub/wheel bearing/brake sensor assembly. Previously quite expensive, these hubs have come down to the $100 to $150 range for a name brand part (donít go cheaper). Installation requires a little over an hourís labor, some large wrenches/sockets, and a few choice words.