When replacing a master cylinder, loosen the soft metal line nuts, then unbolt the master cylinder from the power booster. Pull it of the studs, then use it as a handle to bend the lines upward a little. That will prevent the brake fluid from running out of those lines. Since the nuts are loose already, you won't have to grunt so hard to get them loose while holding onto the master cylinder. Remove the lines then remove the master cylinder. Be aware that brake fluid that drips onto painted surfaces will eat that paint.
Bench-bleed the new master cylinder, then screw the lines onto it. Use it as a handle again to bend the steel lines back down, then bolt it to the power booster. Have a helper push the brake pedal half way to the floor, very slowly. It should take him about 15 seconds to get half way to the floor. As he does that, you'll see air bubbles coming out by the line nuts. Have him holler when he reaches the half way point, then it's important to hold it there until you have both line nuts tight. Holler for him to release the brake pedal quickly. Now loosen the line nuts again about a quarter turn, then have the helper run the brake pedal half way down again. Tighten the nuts, then let him release the pedal quickly. If you still saw any air bubbles, do that a third time.
There will be some air bubbles in the ports of the master cylinder, but there's no way air could be in the steel lines. The purpose of pushing the pedal slowly is brake fluid will get pushed down the lines, but the air has time to float back up. Releasing the pedal quickly washes those air bubbles into the reservoir along with the brake fluid that's rushing back.
I bleed just one line at a time when I do this, so it takes a little longer, but that is just my preference. I can also do it myself, without a helper, especially when I do it at home on my older cars. In most cases I just bolt on the bench-bled master cylinder, then go out and drive the car. The little air by the ports will never go down to the wheels, and after a few brake pedal applications, any hint of soft brake pedal will be gone.
If you start with the lines tight and have a helper run the brake pedal, and you are able to watch in the reservoir, you'll see the air bubbles flow in when he releases the pedal. Most newer reservoir designs don't lend themselves to being able to see the fluid return ports, but this works real well on older cars.
More problems are actually caused when people think they have to bleed at the wheels. Instead of the air taking the short, easy way out to the reservoir, it gets pushed all the way to the wheels. Some cars, like the older Chevy Chevettes, have high points in the lines where the air will remain and never be bled out unless the car can be flipped over like was done on the assembly line. You really want to avoid pushing air all the way through the system on cars like that. When the car has four-wheel anti-lock brakes, brake fluid flows through some chambers that have valves at the high points. They were designed that way on purpose so the air could be bled out when those valves open. They are commanded open by the computer during the bleeding sequence with a scanner, but that has to be done when the brake fluid is able to flow unimpeded to the two wheels. That means those two bleeder screws have to be open during the entire procedure until the air bubbles show up and are expelled.
A lot of older trucks had rear-wheel anti-lock brakes. Those have a different type of "dump valve". It is always open to fluid flow, so no special bleeding procedures are required with those.
A lot of Ford products have four steel lines leaving the master cylinder. The same procedure works, but you can't bend all four lines, so there is going to be more air that needs to be bled out. Don't try to bend any lines after they've been unbolted from the master cylinder. By that time the brake fluid has already dribbled out, so there's nothing to gain by bending them after the fact. When it comes to reconnecting them to the new master cylinder, you'll be in for a lot of frustration trying to get the angle right. Those nuts are real soft too and are easily damaged. If they're lined up correctly, you can run them in about three revolutions with your fingers. If you can only get them about a half turn, then you need a wrench, they are cross-threaded.
When I did this trick at the dealership, we always had one or two young, inexperienced mechanics who were just starting out. Those were the people I grabbed to be my helpers, and it was a perfect time to explain why they were to never push the brake pedal over half way to the floor. This doesn't apply to a master cylinder that's less than about a year old, but it is good to get in the habit of being careful.
For my next comment of value, when you're just starting this job, make a note of the brake fluid level in the reservoir. That is how high you want to fill the reservoir when you're done. The fluid level starts out at the "Full" or "Max" level, then, as the brake pads wear, the pistons move out of the calipers to self-adjust. Brake fluid leaves the reservoir to fill in behind the pistons. There's enough fluid in the reservoir to accommodate all the front pads wearing down to where their metal backing plates start to grind on the rotors. During the routine brake job, the pistons have to be pressed back into the caliper housings to make room for the new, thicker pads. Doing that pushes all that brake fluid back up into the reservoir. Had someone filled the reservoir previously, that fluid is going to overflow and make a horrendous mess. That's why we never top off brake fluid during other routine services such as oil changes.
As long as I'm sharing all this wondrous wisdom, allow me to add one more important comment for anyone researching this topic. When working with brake fluid, it is extremely critical that not a single hint of petroleum product gets into the fluid. A single drop of engine oil, transmission fluid, power steering fluid, or penetrating oil will contaminate the entire system. Rubber seals will swell and become mushy within about a week. The first symptom is the lip seals in the master cylinder will grow past the fluid return ports and block them. You'll be able to apply the brakes, but the brake pedal will be too high and too hard. Heat generated in the brake pads will transfer into the fluid, causing it to expand. Since it is blocked from returning to the reservoir, that expanding fluid applies the brake harder, more heat is generated, the fluid keeps expanding, and this has been known to get so bad the vehicle won't move.
The only acceptable repair for contaminated brake fluid is to remove all the parts that contain rubber parts that contact the brake fluid, flush and dry the steel lines, then install all new rubber parts. That includes the master cylinder, the rubber bladder seal under its cover, calipers, wheel cylinders, rubber flex hoses, and the combination valve on the frame rail under the master cylinder. Most trucks and minivans have a wide range of loading possibilities, so they use a height-sensing proportioning valve at the rear axle to adjust rear brake power accordingly. Those have rubber o-ring seals, so they must be replaced too. If the vehicle has anti-lock brakes, the dump valve or the hydraulic controller must be replaced because it has seals and o-rings. The cost of all those parts can be more than what many cars are worth. If any rubber part is not replaced, the contamination will leech out of it and recontaminate the new brake fluid.
People have been known to use penetrating oil to loosen rusted rubber flex hoses from the steel lines they're attached to. You may get lucky if that is washed off right away, but once you see how freely penetrating oil flows, it's easy to see that it is likely to sneak inside the steel line. Back in the '80s, and before, when cars had front wheel bearings that we greased during a brake job, some mechanics would thoroughly wipe the grease from their hands, then shortly after that, they'd poke the rubber bladder seal back into shape under the reservoir cover. The grease residue on their fingers was enough to contaminate the brake fluid. Brake system specialist even wash their hands with soap and water before touching rubber brake parts they're about to assemble to avoid getting fingerprint grease on them.
The importance of this contamination can't be over-emphasized. When I worked for a mass merchandiser in the mid '80s, we were told of a mechanic at one of our shops in a different state, where he needed to refill the "bleeder ball" they used for quickly power-bleeding brake systems. That tool holds four gallons of brake fluid and is filled from a five-gallon pail. He grabbed a plastic funnel used during some oil changes, wiped it clean, then used it to fill the bleeder ball. The tiny amount of oil residue in that funnel contaminated all four gallons of fluid. The store ended up paying for complete system repairs on over a dozen cars. There may have been more out there that didn't develop systems or the owners didn't notice a problem.
Those are extreme examples of what can happen. Even when we think we're being careful, consider that you're unscrewing steel lines from the old master cylinder in an area where leaking engine oil can spray all over under the hood, and you were just touching those parts. When bench-bleeding the new master cylinder, it is clamped in a vise, and that vise has been used in the past when working on greasy parts. The workbench is usually full of oil and grease residue too.
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Thursday, February 15th, 2018 AT 5:21 PM
