All new breaks, pedal goes to floor

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My husband, who is a retired mechanic, just put all new brakes on his truck. He had me help him bleed them but they still go to the floor. He ranting and raving and no thinking straight! What could be the problem?

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Thursday, May 28th, 2015 AT 7:32 PM

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Most likely the master cylinder was damaged. I'm sure he knows you don't have to bleed the brakes unless parts containing brake fluid were replaced. One of the many mistakes do-it-yourselfers make is they pedal-bleed with a helper who pushes the pedal all the way to the floor. Brake specialists will never go more than halfway down with the pedal. Crud and corrosion build up in the lower halves of the two bores where the pistons don't normally travel in the master cylinder. Running the pedal all the way to the floor, either when bleeding, when pumping the pedal to run the pistons out of the calipers, or when the driver is surprised by a ruptured hose or steel line, runs the pistons over that crud and can tear the rubber lip seals. Most of the time that results in a slowly-sinking brake pedal that doesn't show up until two or three days later.

There can also still be some air in the lines but you'll have to tell me exactly which parts were replaced and what has been done so far to solve this.

If the truck is jacked up with the rear axle hanging down, the height-sensing proportioning valve will limit how much pressure can build up for the rear brakes. That can make it nearly impossible to get the air out of the rear hydraulic system.

Also, does the truck have anti-lock brakes? If so, rear-wheel ABS or four-wheel ABS? A lot of systems require a scanner to tell the computer to open some valves so the fluid and air can get through. I have a number of easy tricks to avoid getting air in the system, both when doing routine brake service and when replacing the master cylinder. I never have to bleed at the wheels when replacing the master cylinder, (except on some Fords with four steel lines coming out of it).

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Thursday, May 28th, 2015 AT 8:05 PM

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