Piston Slap: Copper and the Acura’s soft brake pedal?

PIston Slap Acura brakes lead

Jim writes:

I have a braking issue with my 2007 Acura RDX and had the brake system bled numerous times. After a few thousand kilometers, the brake pedal loses its firmness. A quick pump or two restores the firmness temporarily but it soon fades. I changed the master brake cylinder next, and it solved the problem for a long while (about 14,000 km). I’m back to square one again with a soft brake pedal. Thanks in advance for your help.

Sajeev answers: 

This is a common problem with anti-lock braking systems over time, either with parts aging or a lack of maintenance. That last bit is important, as Acura recommends owners “replace the brake fluid every 3 years.” Whether or not that was done in the past is irrelevant, as your ABS modulator assembly has likely failed and causing the pressure issue you’ve experienced.

2007 Acura RDX Acura ABS Modulator Assembly

The modulator is the heart of your anti-lock brake system, as it meters brake fluid to all four wheels to ensure your brakes (and often traction control) work as intended. Aged-out brake fluid can damage this module, and weak pedal feel is likely because the accumulator can’t collect brake fluid like it once did. The problem likely comes from two contaminants in brake fluid: Water (i.e. moisture) or copper. Most of us know that brake fluid is hydroscopic and just loves to suck water out of the air. But I’ll bet I’m not the only person that didn’t know about copper contamination and how it can affect the precise metering skills of the ABS modulator.

Copper finds a way to reduce the effectiveness of modulator’s pistons, valves, and who knows what else in there! And the copper winds up there because it is present in the brake lines. The Internet says that NHTSA did some in-depth research on the matter of contamination, but all I could find was a document outlining new guidelines for testing brake fluid.

Perhaps calling it a “good read” is overkill, but Page 20 shows that copper contamination is a real problem and that the auto repair industry does brake fluid tests for good reason.

Regarding your need to fix the issue, it’s all about cost-benefit. The replacement part is painfully expensive from Acura and not terribly cheap as a used part. That said, I am not suggesting you need to get this work done. At least not yet. Instead I suggest you speak to a Honda specialist, be it a dealership or a trusted local mechanic. Tell them what you told me, discuss the condition of the brake fluid and the necessity of changing the ABS modulator.

I don’t want to be the reason you fork over a ton of cash, I just want to point you in the right direction to get a firmer brake pedal back in your RDX. Best of luck in the repair!

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