There is a couple of things to be aware of. If you drive a vehicle with a low pedal, you obviously are not going to be able to make the brakes lock up, so the ABS system will not kick in. A lot of people mistakenly think doing so will open the two valves and bleed the air out, but where is it going to go? During normal operation, the brake fluid just vibrates back and forth a little. It does not actually go anywhere, so neither will the air.
The first problem is there was no need to bleed anything just from replacing pads. We pry the piston back into the caliper housing before removing the caliper. (If you need to resort to that dreaded c-clamp, replace the calipers. The piston is sticking on debris in the bottom and is not going to apply or release properly).
The next common mistake is since the pistons were fully retracted to make room for the new, thicker pads, they have to be run back out with the brake pedal to complete the service. Most people push the brake pedal all the way to the floor to do that, and when pedal-bleeding with a helper. Every experienced mechanic can relate his horror stories about damaging a master cylinder by running the brake pedal too far. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel in the master cylinder. Pushing the brake pedal over half way runs the rubber lip seals over that crud and can rip them. That will show up as a slowly-sinking brake pedal, and that often does not show up until two or three days later. This typically doesn't apply to a master cylinder that's less than about a year old
The third thing is it is not necessary to bleed at the wheels when replacing a master cylinder. If you avoid bleeding, you will not run any air down to the ABS hydraulic controller where it gets trapped. This takes a little more work on Fords that have four steel lines coming out of the master cylinder, but on other cars, the master cylinder can be replaced in about ten to fifteen minutes.
For my final note of magnificent wisdom, most cars today require the use of a scanner to command two valves to open in the ABS hydraulic controller. You need to have two bleeder screws open so the brake fluid can flow out and take the air with it. Once it's connected and the bleeders are open, the process takes all of about five seconds.
There is one more thing worth mentioning, but I do not know if it applies to this vehicle. This applies to only GM front-wheel-drive cars, not any other brand I'm aware of. When you try to pedal-bleed with a helper, the first bleeder screw you open will trip a valve inside the master cylinder as soon as the pedal is pushed. From that point on, you will not get any fluid to flow from that wheel or the one in the opposite corner. If the left front won't flow any fluid, the right rear will also not flow. You typically have to press the brake pedal more than half way to the floor to trip that valve. There are misinformed people who still think tripping that valve can be avoided by bleeding the wheels in a specific sequence, but that is not true. This feature prevents total loss of brake fluid in the event of an external leak, such as a ruptured rubber flex hose. It wouldn't make sense to say to start with the right rear wheel because who knows which hose will pop first? If the right rear hose ruptures, you better hope that valve trips to stop fluid loss. That valve does not know the difference between someone bleeding the system or a leak. No matter which wheel you start with, that valve is going to trip. A lot of do-it-yourselfers replace the master cylinder for that failure to flow fluid to two wheels, then have the same problem with the new one after pedal-bleeding again. I only use gravity-bleeding. It doesn't take too much longer, and it avoids this problem. Some mechanics like to start with the rear wheels to get as much air out as possible on the first one. Some like to start with a front wheel because being such short lines, it takes very little effort to get one line fully bled. In doing major brake work since the mid 1980's, when bleeding is necessary, I always start with the wheel I'm standing closest to.
To avoid all this heartache and frustration, professionals never push the brake pedal over half way to the floor.
The only reason to bleed at the wheels is every car manufacturer recommends replacing the brake fluid periodically. That is to get out the moisture it naturally absorbs. That moisture is part of what causes the corrosion in the master cylinder and calipers, and it lowers the boiling point of the brake fluid. Few of us actually do that service, but it is a valid reason for bleeding.
Here is links to a couple of guides that might offer more information:
https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system
https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor
Tuesday, October 23rd, 2018 AT 10:00 PM
