Whether you're bleeding with a helper or surprised by a sudden leak, pushing the brake pedal to the floor will usually damage the master cylinder. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Running the pistons all the way down tears the rubber lip seals on that crud. Chunks can break off and intermittently block the fluid ports, and the torn seals will cause a low or slowly-sinking pedal. Torn seals won't push any brake fluid out either.
To prevent this, I only use gravity-bleeding. If you have to resort to pedal-bleeding, never push the pedal more than halfway to the floor. The exception is if the master cylinder is less than about a year old because it will not have developed that corrosion yet.
Mar 15, 2021 at 7:22 PM
