Random fast pumping of the brake pedal when stopping

2001 CHEVROLET IMPALA

130,000 MILES • 3.4L • 6 CYL • 2WD • AUTOMATIC

RAY DICKERSON2

MEMBER
2 POSTS

Intermittently, when depressing the brake pedal, the pedal pumps very fast and you can hear a grinding sound coming from what seems to be the brake pressure modulator valve. This does not happen all of the time, maybe one in ten times when using the brake.
Any ideas?

Jun 3, 2017 at 7:20 AM

CARADIODOC

✓Experienced Mechanic
34,403 POSTS

That is called "false activation" of the ABS system. GM has had a real big problem with their front-wheel-drive front wheel bearings developing a little normal slop in as little as 15,000 miles. The built-in wheel speed sensors develop rather wimpy signals to start with. Add that slop in the bearing, and the signal can drop to such a low level, the computer cannot read it, and it thinks that wheel stopped rotating.

To identify this, watch the wheel speeds on a scanner, during a test-drive. You will see one of them drop to 0 mph when the activation occurs. That typically occurs below about 20 mph.

Mar 12, 2021 at 3:12 AM