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Spongy brake pedal on 2017 GLC300 - Time for a brake fluid flush?

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Chris P. · rep 0 · May 22, 4:19 AM · 1 replies · solved · Question

Vehicle: 2017 Mercedes-Benz GLC300

Hey everyone,

I own a 2017 Mercedes-Benz GLC300 with about 55,000 miles. Lately, the brake pedal has felt a bit spongy, and I feel like I have to press it down further than before to get the same braking power. The car stops fine, but the feel isn't as firm as it used to be.

I checked my service records, and it looks like the brake fluid hasn't been flushed since I bought the car three years ago. Is this a common symptom of old brake fluid? What's the recommended interval for a brake fluid flush on these GLCs? Any tips for a DIY flush, or should I take it to a shop?

Thanks for the help!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:19 AM

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Hello Chris,

Your observation of a spongy brake pedal on your 2017 GLC300, especially given the time since the last service, strongly suggests that it's time for a brake fluid flush. Mercedes-Benz, like most manufacturers, recommends a brake fluid flush every two years, regardless of mileage. This is a critical maintenance item often overlooked.

Why Brake Fluid Needs Regular Flushing:

  • Hygroscopic Nature: Brake fluid is hygroscopic, meaning it absorbs moisture from the air over time. This moisture lowers the fluid's boiling point.
  • Corrosion: Water in the brake fluid can lead to internal corrosion of brake components like calipers, wheel cylinders, and the ABS module.
  • Reduced Performance: When brake fluid boils (under heavy braking, for example), it creates vapor bubbles. Since vapor is compressible, this leads to a soft, spongy, or 'long' brake pedal, significantly reducing braking effectiveness and safety.

DIY vs. Professional Flush: Performing a brake fluid flush on a modern Mercedes-Benz can be challenging for a DIYer due to the sophisticated ABS/ESP systems:

  1. Proper Bleeding Procedure: A specific sequence must be followed for bleeding each caliper.
  2. STAR/XENTRY or Advanced Scanner: To properly bleed the ABS/ESP hydraulic unit, especially if air has entered the system, a diagnostic tool is often required to cycle the ABS pump. Without this, air can remain trapped, leading to persistent sponginess.
  3. Correct Fluid Type: Use only DOT 4 PLUS (MB 331.0 specification) brake fluid. Never mix different types of brake fluid.

When to See a Shop:

  • If you're not experienced with brake system maintenance or don't have the necessary diagnostic tools.
  • If the pedal remains spongy after a DIY flush, indicating trapped air or a more serious issue like a failing master cylinder or brake line leak.
  • There are no direct OBD-II codes for old brake fluid, but if the ABS/ESP system detects a fault during a DIY bleed, it could trigger codes like C0040 (Brake Fluid Level Sensor Circuit) or other ABS-related codes. A professional will have the tools to clear these and perform a proper system bleed.

We highly recommend having a professional perform this service to ensure your braking system is safe and performing optimally.

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