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Brakes drift to the floor

Tiny
MADLENEPGH
  • MEMBER
  • 1997 GMC SONOMA
  • 4.3L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 127,000 MILES
Brakes have been drifting to the floor for the last three weeks or so. When I am at a stop sign I feel like the truck is drifting so I have to apply the brakes even more and they are going down toward the floor. Over a three week period the fluid level was maintained, lines underneath were all checked and no leak found, and master cylinder was suspect and replaced with a new master cylinder. But the brake pedal still drifts to the floor. What do you think?
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Thursday, October 19th, 2017 AT 6:07 PM

1 Reply

Tiny
STEVE W.
  • MECHANIC
  • 12,958 POSTS
Brake pedal drop only happens with a fluid leak, either external though a bad line/wheel cylinder or caliper seal, or internal through a bad master cylinder or fluid bypass into the booster through the master cylinder.

If you are not losing fluid but the pedal is still dropping you either have a second bad master cylinder or possibly a bad rubber line that is acting like a balloon.

Air in the system would create a mushy pedal.
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Thursday, October 19th, 2017 AT 8:18 PM

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