mjp1954

Any help diagnosing a brake pedal that slowly sinks at a stop light? No obvious leaks at the wheels, brake lines or master cylinder.

ibidu1

iTrader: (1)

When was the last time you flushed the brake fluid? If you never did, I would do this first. But, the abs brake system is not as easy as the standard brake system. To do a proper brake fluid flush you need a computer scan tool to release the abs pump.

What I did, and its super quick and easy! I grabbed a turkey baster, poured the old fluid in a empty water bottle, opened the top cap. I siphoned the brake fluid out of the reseviour. Then poured in fresh fluid, after 3,000 miles I would do the same thing and would repeat the same thing 3 times over. Eventually the fluid will all get replaced and come out clean. My first time I did this, the fluid came out so nasty, I too had the brake pedal slowly drop at stop lights.

mjp1954

Thank you for the reply. It looks like the previous owner did the brakes and fluid about a year ago according to records. Is the master cylinder leaking internally if no leaks are externally seen?

LeX2K

Master cylinder can be faulty and bleed pressure internally causing a sinking pedal. In rare cases a bad brake booster will cause a sinking pedal a member here recently had this happen.

ibidu1

iTrader: (1)

I wouldnt trust the records, and examine the brake fluid yourself. Open the cap and inspect the color of the fluid. A shop can easily claim to have done a fluid change, and the tech wouldnt have bothered because of how difficult it is to properly bleed the brakes with the abs system.

If the fluid looks great, then you can suspect the master cylinder is bad. To rule out the brake booster, you would hear an air leak whissing sound when you press on the brake pedal.