Auto Repair Help
LANCE’S CORNER – AUTO REPAIR HELP

DIAGNOSE – BRAKE PEDAL PULSATES WHEN I STEP ON IT
by Lance Wright


SYMPTOM SUMMARY

The brake pedal pulsates when applied. The symptom may be worse at higher speeds.

USUAL CAUSEbrake_pedal
This symptom is usually caused by warped brake rotors. If the vehicle is equipped with four wheel disc brakes, it could be either the front or rear brake rotors. Excessive heat build up on the brake rotors can cause them to warp , causing a brake pedal pulsation. This can be the result of heavy stop and go driving. If the rear brakes are not adjusted properly, the demand on the front brakes will be higher which can also contribute to front rotor warpage. Some vehicle brake designs are also more prone to warpage than others. A warped brake rotor will cause the piston in the brake caliper to pulsate in and out when the brakes are applied. This in turn, causes the hydraulic brake fluid to transmit this pulsation to the master cylinder, then to the brake pedal.

DIAGNOSIS
If the symptom only occurs during braking and is worse at higher speeds, the most probable cause is warped brake rotors.

CORRECTIVE ACTION
The corrective action is to replace or have the rotors machined (often referred to as surfaced or turned). It is recommended that the brake rotors be machined during replacement of the brake pads. This provides a parallel flat mating surface to the new pads. If the rotors are being machined to correct the pulsation, it is also recommended that the pads be replaced unless there is more than 50% life left on the existing pads.

PRECAUTIONS, TIPS, and NOTES
Brake rotors become extremely hot after vehicle operation and braking. Use caution when working on the braking system. Brake rotors have a manufacturers minimum thickness. Never machine a rotor beyond the minimum thickness. This will only contribute to additional warpage, since the rotor cannot adequately dissipate the heat generated during braking. The minimum thickness is usually stamped on the brake rotor, usually on the back side. The brake rotor should be replaced once it has reached or has become thinner than its minimum thickness.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)

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3 Comments on "Brake Pedal Pulsates When I Step On It"

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2 years 11 months ago

Never overlook water-laden brake fluid as a cause of pulsation. Dot 3 Brake fluid should be changed at least every 2 years.

2 years 5 months ago

What is the cause of a thinning brake rotor?

Jack
2 years 4 months ago

Hi, i have a 1999 f150 i replaced the front rotors and pads and truck still has pulsation when braking. any tips what can be the problem.

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