Both. If the truck has four wheel anti-lock brakes, you may need a scanner to open some valves so air can be expelled from some chambers during bleeding. Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons do not normally travel. When the brake pedal is run all the way to the floor, either during bleeding, or when surprised by a sudden leak, the lip seals are run over that corrosion and can be torn. That results in a slowly-sinking brake pedal, and that often doesn't show up for two or three days.
Master cylinders are most commonly damaged from that crud when the hydraulic system is pedal-bleed with a helper, by do-it-yourselfers. To avoid this problem, professionals rarely pedal-bleed, and if they do, they do not run the pedal more than half way to the floor. We typically prefer to gravity-bleed. That requires no special tools and no helper.
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Thursday, March 16th, 2017 AT 2:48 PM
