So much has been said about bleeding brakes it could fill a book. The purpose of bleeding brakes is to get all traces of air out of the system. Air bubbles act like a spring or cushion which is undesirable and causes a spongy pedal feel or can actually render the brakes incapable of stopping the car if a lot of air is present. Excessive air in the brake system can also increase the time it takes for full pressure to occur between the time your foot hits the pedal and deceleration begins. This may sound like splitting hairs but at 70 MPH a car will travel 102 feet in 1 second, so even 1/4 second delay = 25 feet. 25 feet could be the difference between smashing another car in the rear or killing a pedestrian.
There is no law that I know of that says it's illegal for an unqualified car owner to work on his own brakes but sometimes I think it might be a good idea.
The basic method of bleeding brakes is to have somebody step on the pedal, open a bleeder, brake fluid squirts out (hopefully not in your face) along with air bubbles as the pedal goes to the floor, close the bleeder, let the pedal up. That is about the extent of some peoples knowledge. Many repair manuals suggest putting a clear plastic tube onto the bleeder with the other end submerged in a jar of clean brake fluid, this way you can see the bubbles as they go through the tube, when there are no more bubbles viola! your done. A distinct improvement over a face full of brake fluid not to mention brake fluid stings when it gets in your eyes, trust me, it does! After 20+ years in the automotive trade my eyes have been assaulted by every kind of fluid found in an automobile. Gasoline is the worst.
What about when the entire brake system is dry as in filled with nothing BUT air? How do you proceed? Where do you start? Using the basic method will work but you'll be at it for 2 days and go through about a gallon of fluid by the time you get to some semblance of a decent pedal, if ever.
Most of the shops I worked for had a ìPower Ballî bleeder. This is a spherical tank on wheels with a gallon or so of brake fluid and a hose to deliver the fluid to the master cylinder under pressure and an assortment of attachments designed to fit a wide variety of master cylinders usually those found on the kind of cars the particular shop caters to, like a Ford dealer will have caps to fit every Ford ever made etc. The power ball has an air fitting like a tire and you charge it up with an air chuck to about 15 PSI as I recall, fill the master cylinder, attach the correct cap from the power ball and open the valve and go around from 1 bleeder to the next til all the air is out and you're done. It's best to use a short plastic hose on the bleeder so you can catch the fluid and not make a mess.
This involves letting gravity do the work instead of stepping on the brake pedal. The nice thing about this method is you can do it without a helper. This may be hard to get started if the system is totally empty due to no siphon action, remember you're using gravity which requires a differential in height. But for a car that has a pedal and you want to just do some maintenance bleeding this works great. The nice thing about it is the bubbles are not compressed from stepping on the pedal.
Caution: The threads on the bleeder aren't sealed, so when you have the tubing on the bleeder into the jar with the bleeder open, it is also acting as a siphon and air can enter through the bleeder threads and appear in the tube and you will see bubbles forever but they aren't coming from the system.
This involves a little vacuum pump you attach to one of the bleeders and virtually suck the fluid down from the master cylinder. IMO this has to be the stupidest idea yet because it has the risk of sucking air into the system from the master cylinder and the brake calipers.
This involves the use of clear plastic tubing long enough to reach from the farthest bleeder back to the master cylinder and after hooking up the hose to the bleeder put the other end into the master cylinder (have a friend hold it or you can use a clothespin) and slowly work the brake pedal till you see no more bubbles. Have something handy to stick in the tube when you remove it so you don't lose the fluid in the tubing. Repeat for the next wheel.
It's common knowledge to start at the bleeder farthest from the master cylinder and work your way closer to it. In 20+ years working as a professional auto mechanic I've bled the brakes on more than 1000 cars. I've used the far to near method, the near to far method and no order what-so-ever, the results were exactly the same so I have to conclude that this ìcommon knowledgeî is nothing but an urban legend. Think about this: Suppose you have a 1/2 teaspoon of air in each caliper, what does it matter which wheel you do first? Is the air from one caliper going to travel to another caliper? That's ridiculous!
There is a small hole in the MC just in front of the primary cup which is how the fluid gets from the Reservoir to the pressure cylinder. As you begin to depress the pedal this hole is closed off by the primary cup with less than 1/4 inch of movement. Once the hole is closed off pressure can be built up to operate the brakes. If you look into the reservoir you will probably be able to see this little hole. When you install a new or rebuilt MC, you need to fill the pressure cylinder with fluid before you ever step on the pedal. Failing to do this simple step will push a big gulp of air into the system and you will be fighting for hours to get the last trace of it out. Some people recommend ìBench Bleedingî the MC before its installed. This process will fill the pressure cylinder before you install it. There is a much easier and neater way to fill the pressure cylinder.
To start with make sure the handbrake is on before you remove the MC.
Install the MC, hook up the brake line, and put some fluid in the MC. Now have a friend look inside the reservoir as you push the pedal NO MORE THAN a 1/4 inch. Every time you do this 1 bubble of air will emerge from that little hole. Keep doing this until no more bubbles come out. Now depress the brake pedal about halfway down and hold it there for 30 seconds, then let it up and release the hand brake and repeat the tapping procedure again. Repeat this until you can't get any bubbles out of that little hole. Top up the MC and you're done and you probably wont even have to bleed the brakes at all.
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23 January, 2013