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Less Brake Power At Red Light w/ AC On?

I feel recently that when I am sitting at a red light with my foot on the brake, and the AC blasting, that sometimes the car begins to creep forward until I push harder on the brake. At times, the amount of pressure on the brake pedal is fine to hold the car still, but other times, it isn’t.

What might the issue be?

Your power brake booster is failing.
Get it replaced before you lose all power assist for the brakes on your mystery vehicle.

Is this something generally covered under warranty (07 Kia) … what might I go in there telling them? (exactly what I told you?)

If your mystery vehicle was built during the 70s or 80s I could understand this, but if you’re driving something newer it shouldn’t be happening.

When the AC compressor cycles on it requires more engine power, and in older cars the idle speed may have been increased to compensate, thereby requiring you to apply more pressure to the brake pedal to overcome the additional engine power.

Please tell us the year, make, and model of your vehicle.

The problem might be with an internal leak in the brake master cylinder.

When the AC is on it raises the underhood temperatures. This heat can effect the brake master cylinder where the brake fluid starts to bypass the cup seals and the hydraulic pressure is reduced and the brakes release slightly. The vehicle then creeps forward until you step on the brake pedal a little harder.


Tell them exactly what you told us.

Then be prepared for them to tell you they can’t duplicate the problem.

'07 Kia WHAT? Kia makes more than one model. Also, tell us which engine your '07 Kia has. We’re not mind readers.

The Bumper-to-Bumper warranty coverage on your car–to the best of my knowledge–is 3 years/36k miles (whichever comes first) on every part of the car. If this is a 2007 model, you are past either of those milestones.

The other warranties on your car (100k Powertrain Warranty + Federal Emissions Warranty) will not cover components of your brake system. Ergo, this repair is not covered by any of your past or present warranties, at least to the best of my knowledge. You can double-check my opinion by referring to the documentation in your glove compartment.

All of that being said, there is no reason to take this car to a dealer, unless you want to pay an inflated price for what appears to be replacement of the power brake booster. Any competent mechanic can do this job.

Just describe the symptoms accurately, and–for your own good–do NOT take the car to a chain operation like Sears, Midas, Meineke, Monro, or Pep Boys. A local independent mechanic can take care of this repair at a lower cost than either the dealer or a chain operation.

2007 kia spectra5, 138-hp, 2.0-liter I-4 (regular gas), 4-speed automatic w/OD.

Curious if there’s anything I can look at before I take it somewhere.

If you have a leak in the master cylinder, the fluid level should be low. Check your brake fluid level.

If you have an internal leak in the master cylinder the fluid level won’t drop because there’s no external leak of brake fluid. An internal leak means the cup seals within the master cylinder are leaking and no longer sealing.