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The handbrake indicator light on my 2005 Scion tC intermittently turns on while I'm driving.

I've read about two causes: low brake fluid and (not likely) the handbrake is somehow getting lifted while driving.

Reason I say the second cause is unlikely is because the handbrake light does not turn on until I lift the handbrake far enough for the first click, and you have to lift it pretty far up for just that first click. While driving, the handbrake is completely down.

As far as my brake fluid being low, I'm not convinced. Obviously, I can't check the fluid while I'm driving and pressing the brake pedal, but while the car is stopped, the brake fluid levels are in the normal range. I replaced my front brake pads two months ago and my rear brake pads just last week.

Could there be any other reasons for the handbrake indicator light turning on while I'm driving?

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Can you confirm from your owner's manual whether the handbrake light is also used as a brake fluid warning light? – William Cline May 26 '11 at 23:41
Yes, p. 95: "(a) Brake System Warning Light. This light comes on in the following cases when the ignition key is in the "ON" position. * When the parking brake is applied... * When the brake fluid level is low..." – tgxiii May 27 '11 at 3:56
up vote 2 down vote accepted

Toyotas (and presumably Scions as well) are VERY sensitive to brake fluid level. You can be well within the "acceptable" range, yet when under braking the fluid will move enough to just barely trip the sensor. I keep mine topped right up to the "full" mark to avoid the light flickering under braking. Then, when installing new brakes, I bleed some fluid out first to prevent overflow when the pistons get pushed back in.

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I'm inclined to accept this as the right answer. It reflects a lot of what I've read on Scion forums as well. – tgxiii Jun 29 '11 at 20:24

I had a similar problem a couple of years ago which turned out to be the handbrake cable being loose - at the rest position it was slightly slack which enabled the lever to 'bounce' slightly, which made the warning light come on. It was solved by disconnecting and refitting the cable, then re-tensioning it.

If you can get at the switch on the lever, try disconnecting it and see if the problem persists. If it goes away, then the handbrake is the problem, and if it does not then it is likely to be the fluid warning sensor (obviously re-connect it once you've done this test!)

I have never come across a situation where brake fluid could be low when driving and normal when stationary, usually if it gets low it stays low! Have you tried getting an assistant to press the brake pedal while you observe the fluid reservoir?

With most braking systems I have come across (and I have no reason to believe Toyota's is any different), as you press the pedal the piston closes off the reservoir first, then begins to push the fluid into the system. You'll see this if you ever bleed the brakes as the level in the reservoir will only drop when you release the pedal. Therefore, flid cannot get directly from reservoir to system, or vice-versa, and only one master-cylinder-full can be transferred at a time.

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Thanks Nick. I'll try this. – tgxiii May 27 '11 at 15:05

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