How to bleed brakes

[bill buttermore]

The idea is to remove any air bubbles in the brake lines from the master cylinder to each wheel cylinder (rear wheels) or caliper (front wheels).

These directions presume you have a good master cylinder or a new one that has been bench-bled. Brakes are bled with the engine off.

You will probably need to remove the rear wheels to gain access to the wheel cylinder bleeders. You may be able to get at the front bleeders on the calipers by turning the wheels all the way left or right. I find bleeding is a lot easier with all the road wheels removed and the car up on stands. Never get under a car supported only by a jack!


Start by filling the master cylinder reservoir and replacing the cap. Don't let brake fluid be exposed to the air - it will absorb moisture from the atmosphere which can cause troubles in the brake lines. What you want is to have a helper pump the pedal a couple of times (press down - release; press down - release) then press down and hold. While he is maintaining pressure on the pedal, you open the bleeder valve on the wheel cylinder or caliper and relieve the pressure by releasing fluid from the bleeder valve. The brake pedal will suddenly go down, at first nearly all the way to the floor. The helper must not allow the pedal to come up while the bleeder valve is open or air will be sucked in through the bleeder valve. At the wheel, when the fluid flow stops, close the bleeder screw and instruct your helper to release the pedal, then "pump and hold" again. After a few times, depending on how much air is in the lines and where the air is located, you will notice as you open the bleeder valve, that the fluid flow is interrupted by escaping air. It kind of makes a "spitting" sound. That is good - that's the air you are trying to remove. Keep repeating the procedure until the stream of fluid is solid - no air - and clean in appearance.

While you are bleeding each wheel, after about five or six squirts, check the level of fluid in the master cylinder reservoir. If you let the reservoir go empty, you will introduce air into the lines and you will have to bleed the master cylinder and all the brakes again.

I like to place a box wrench on the bleeder screw nut in a position that allows me to both open and close the valve (hopefully) without repositioning the wrench. I then attach a length of flexible vinyl tubing (maybe 3/16" inside diameter) that fits tightly over the end of the nipple on the end of the bleeder. I run the hose into a plastic pan to catch the fluid that comes out. Be careful, brake fluid removes paint. Also, don't use more than a couple of foot-pounds of torque to close these little bleeder screws - they are not solid and they can break off very easily.

There are a couple of inexpensive one-man methods. You can use a vacuum pump to suck out the air from each line, but the problem with that is that you have to open the bleeder screw to open the line and when you do, you usually end up sucking some air through the threads on the bleeder screw making it difficult to know when you have all the air out. An older method is to take the hose described above and put it into a (clean and dry) coke bottle with the end of the hose submerged under a couple of inches of brake fluid. This is not a bad idea, even with a helper, cause if your helper screws up and releases the pedal when the valve is still open, it will suck mostly fluid in from the bottle instead of air. The problem with this method is the same as with the vacuum pump. That is, on the release stroke, if the valve is open, air can be sucked into the system through the threads on the bleeder screw.

Be careful if your bleeder screws are corroded. They will break off in the cylinder or the caliper with surprisingly little force applied, and then you have a real piece of work on your hands. A good plan is to squirt around the bleeder screw (not into it) with a good penetrating oil like power blaster, kroil, or sea foam deep creep (WD-40 is not very good for this). Remove the cap from the screw and give it a couple of taps square on the end of the screw with a small ball-pein hammer, and squirt again to encourage the oil to penetrate the threads. Best if you can let it sit overnight, but give the oil at least 30 minutes to work before you try to open the bleeders.

If your bleeders don't have the little rubber caps on them, make sure the passage is clear to let the fluid out. Sometimes you have to clean them out with a small drill bit.

The traditional method is to start at the wheel farthest from the master cylinder, usually the right rear, and work to the shortest run, i.e., the left front. As you work your way around, the pedal should become firmer and progressively higher from the floor. When you are done, if you have removed all the air, the pedal should be firm, there should be no "sponginess," or softness when depressed.

[carl]

Nice write up on this, very complete.

The whole idea of using clear vinyl tubing on the bleeder is that its a pretty good indication for telling when things are primed and all the air bubbles are likely out.

To minimize cluster, spills and contamination: To seal the end of the vinyl tubing from drawing back air in, I bend/kink the vinyl tube end, then use a pair of small vise-grips to clip onto the bend in the vinyl tubing instead of using the fluid draw/jar. Fluid draw is very good but in my case, if there a possiblity to spill or contaminate, it usually will happen to me. To bad the lotto doesn't work with me like that.

I've also plugged the tubing end with a clean screw to insure no air pulls back, but its not necessary and overkill if kinking/using vice grips, and likely not good enough if using a plug alone--unless it is a perfect size for the tubing..

A good point (that I've ignored) is that the threads still could suck in air. I wonder if a precise teflon tape-job on the bleeder threads could be a good idea, but we don't want the tape to disintegrate or stray tape shards to contaminate the system.

I've used fish-tank pump clear vinyl tubing but found that it is way too stiff and causes air to seep-in around the bleeder, pops-off very easy, and then stretches out. Some of the very soft vinyl small pneumatic tubing is better and seals nice around the bleeder and is very flexible as not to pop-off during the process. I don't know the integrity of silicone tubing with brake fluid, and if its is a good idea, but should be ok for a short time, as long fluid doesn't get sucked back into the caliper. If using soft silicone tubing, a clean unpainted golf tee can be used to plug the end airtight, and its super quick.

The hydroscopic component of the fluid was another good point that some people ignore especially when its wet or humid outside.

[Tony94]

Start by filling the master cylinder reservoir and replacing the cap.

I suggest removing pretty much all of the fluid in the master cylinder with a turkey baster and replacing it with new fluid. Do not aerate the fluid! You may only be able to get the front brake fluid without some more evolved tool than a baster.

Don't let brake fluid be exposed to the air - it will absorb moisture from the atmosphere which can cause troubles in the brake lines.

I suggest not doing the brake bleeding or a DIY brakejob on a humid day, but I can't imagine that significant amount of moisture would be absorbed by the fluid in a short amount of time, so it's not a "hard" recommendation.

[snipped: general brake bleeding procedure for cars without ABS]

Though I've never done anything special with high-dollar electronic tools, my shop manual says that they are required to activate the automatic bleed sequence or some such thing on my OBD I '94 car. I use a vacuum bleeder. Pressure bleeders are available though at greater cost. I put teflon tape on the top few threads of the bleeder screw which aids bleeding and prevents seizing (rebuilt calipers many times have enlarged threads rather than keenserts or helicoils.

Be careful if your bleeder screws are corroded.

That is a sign of a poorly maintained brake system. If the brakes were bled regularly, the screws should not break or strip.
I do the teflon tape thing, but you have to be careful not to tape over all of the threads so that the teflon gets into the system. [/quote]


Always check for leaks after performing a brake bleed procedure. Also, check for brake drag (a common problem). Rather than bleeding though, I recommend REPLACING (via bleeding) the brake fluid every year. I use synthetic fluid. Finally, fluid WILL gravity feed even on ABS cars so you can utilize that aspect as appropriate.

(The OP and the responses just beg for wiki and apparently Automobile Mechanics 101 IS content here. Nothing in the OP or repsonses is Bonneville-specific nor 1994 Bonnie-specific).