Brake light comes on while driving; 03 Windstar

My 2003 base model Ford Windstar’s brake light is coming on while I drive. When I take my son to school in the morning, the light comes on as I’m braking from about 40 miles an hour at the first couple of stoplights I hit - within about 10 minutes of starting up. After that, it doesn’t come on for the rest of the drive. I don’t hear any brake noise or feel anything different about the brakes. It has about 55,000 miles on it; I bought it used a year ago with about 39,000. I’ll take it somewhere to get it looked at, but in the meantime, any ideas as to what I should be asking the mechanics?

Do you mean the rear brake lights, or the brake indicator light on the dash board?

Sorry, to be clear: The brake light indicator on the dashboard - the same one that comes on when you apply the parking brake.

I had something similar happen on my car last summer.

Basically, I was a little low on brake fluid, and the pads were pretty close to requiring replacement (although they were still usable at the time). The worn out pads meant that the shoes needed to move slightly farther from “rest” to give the same braking power, which required more fluid to be sent to the brakes. Combining this with being a little low on fluid meant that when doing heavy braking, the fluid resevoir dipped below a triggering threshold, which caused the brake light to come on.

You can have the mechanic check your brake fluid (or maybe that’s something that you can check yourself and maybe top it off), and give a look at the brake pads.

Don’t top off brake fluid unless you know what’s going on. If that light is coming on for a reason, say, a leak. Adding fluid will mask the problem.
If it’s coming on becuase the pads need to be change, get them changed, adding fluid in that case will simply make a bigger mess when the mechanic changes the brakes.

Nitpick. Brakes are self adjusting (both front and rear), no matter how worn out (within reason) they will always travel the same distance. This is why if you get your brakes changed before you destroy them, the new ones won’t feel any different then the old ones. You are kinda right. While each pump of the brake requires the same amount of fluid, worn out pads mean more fluid is in the lines and thus there is less in the resivoir.

If I where you, the first thing I would do is go and take a look at the resivoir and see if it’s low. If it is, it’s most likely worn pads.

[Very nitpicky nitpick] The volume of the brake lines is constant regardless of the condition of the pads. As the brake pads wear, The caliper piston is pushed further out and over time the volume of fluid in the caliper increases. This leads to a reduction of fluid level in the reservoir. [/VNN]

Getting back to the OP, I am guessing that the first couple of stops are either facing uphill or down hill, and the balance are on more or less level ground. The brake fluid will slosh and can turn on the low fluid warning light on a hill when it won’t on level ground.
Oh, and make sure you tell them that the brake warning light on the dash is on, when I first read the OP, I thought you were talking about the brake lights on the back of the vehicle. (maybe not enough coffee)

Joey P, I guess that’s why I pay professionals to do anything on my car more complicated than changing the oil: I know just enough about what’s going on to get myself in trouble. :slight_smile:

Rick, that was an excellent guess. The stoplights where the brake light comes on are at the bottom of hills.

Thanks for the advice, everyone. I’ll take it by CarMax (where I bought it) later this week and have them take a look.

It’s a virtual certainty that the light is coming on because of low fluid. It’s not quite low enough to trigger the light standing still, but when it sloshes around (braking, accelerating, and/or cornering) it momentarily gets too low where the float for the indicator light rides. You can verify this by checking the fluid level in the reservoir; it will be right around the “minimum” mark.

95% of the time this occurs because of brake pad wear. As the pads wear and get thinner, the caliper pistons (which self-adjust and always stay right next to the pads) end up protruding farther out of the calipers, and the fluid follows them (this is why there’s so much space between the “maximum” and “minimum” marks on the reservoir). Typically, when the pads are 80-90% worn the fluid level gets low enough to turn the light on, telling you it’s time to have your brakes inspected.

The common procedure on non-ABS systems is to simply press the pistons back into the calipers (to make room for the new pads, which are thicker than the worn ones), and the fluid level will end up back at the “maximum” mark. Topping up the fluid first just means having to extract that extra fluid during the repair. On ABS systems, it is advised NOT to let the fluid back up into the reservoir, for fear that some little debris particle (normal occurrence in old fluid) might clog a critical ABS orifice, perhaps necessitating a very expensive repair. The way to do it is to open the caliper bleeder screw before resetting the piston, and of course adding fluid to restore that which is lost this way.

The other 5% of the time there is a leak. This is potentially a much more serious situation. The fluid level drop is MUCH quicker than with pad wear, and if it gets too low the system will suck air and braking will be compromised. Topping up will buy time and can be helpful. However, if the light just flickers for a number of days before staying on steady, it’s very unlikely that this is the case.

I’m not sure what you mean by “asking the mechanics.” The thing to TELL the shop is the symptom(s) you’ve observed. If they’re competent, that’s all that’s necessary. If they’re not competent, you shouldn’t be dealing with them.

Okay, well, ask, tell, talk to - communicate with them. Cut me some slack here, I got six hours of sleep last night.

Sorry, I didn’t mean to be snarky.

Had this happen before when it was a sensor detecting an electronic problem with the ABS system.

::: Rick whistles Nobody Does it better under his breath, and wanders out of this thread:::