I sit at a job site with my 2009 GMC Sierra idling for heat, or aircon. After 20 minutes of idling, I go to drive off and the brake calipers are locked up. Already replaced pads, discs, short lines, booster, master cylinder, ABS. All that remains original is hard lines. Truck has some rust on the underside. Wondering if the lines are cruddy and act like a check valve. Also wonder if the auto pedal adjust is malfunctioning while idling, filling the lines with pressure which can’t bleed off quickly. After shutting the truck off for 15-20 minutes, the brakes release and all seems fine. Scratching my head!! Is the pedal adjust controlled by one of the computers? Or, could the button be malfunctioning?

  • Welcome to Motor Vehicle Maintenance & Repair! Do you know if your Sierra has hydroboost or vacuum assist brakes? Mar 8, 2021 at 23:56
  • Paulster2, sorry I do not.
    – RayGMC
    Mar 9, 2021 at 0:01
  • How can I ascertain that?
    – RayGMC
    Mar 9, 2021 at 0:01
  • Could they have put the wrong type on when replacing?
    – RayGMC
    Mar 9, 2021 at 0:03
  • Looked up work Midas did on truck 2/3/20. They claimed the previous shop put Hydro boost pads and they replaced with Vacuum boost type.
    – RayGMC
    Mar 9, 2021 at 0:08


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