Bursting the Bubble

Bursting the Bubble
Pump it up! Hold! Release. Pump it up! Hold! Release.” And sol on. How often do you hear that-one guy in the cable car pumping the brake bicycle, the other at a caliper or back plate, box wring on the hemophiliac ? Two people plus their time, all to get rid of what turns out to be a very expensive bubble of air. Of course, many shops use a ability hemophiliac to make the work a one-man occupation, but sometimes you need the greater fluid speed of the pedal to push out that last bubble. What ‘s more, many newer cars use one mannequin of antilock brakes or another, and those systems are much harder-if not impossible-to bleed using conventional methods .
so why bleed the brakes at all if you do n’t have to ? Because you do have to. Remember, brake fluid is “ hygroscopic ” ; that is, it likes water, so you should flush the arrangement at least every other year, according to brake authorities. That ‘s credibly a button-down calculate, because if you live where it ‘s identical wet or the car owner drives in more body of water than most, you may have to flush the fluid more frequently. In addition, an outdoors can of brake fluent, or an uncover master cylinder, or a leak wheel cylinder can draw moisture from the driest breeze in the world and absorb it into the brake system. This has two effects, and they ‘re both bad : First, the moisture can react with the metallic element inside the brake system and let rust and electrolytic corrosion begin. Second, the boggy brake fluid boils at a much lower temperature than it does new from the can. You notice this hygroscopic consequence any clock you get brake fluid on your skin, and that ‘s only a few drops .
This boil point reduction is identical dramatic. Most fresh brake fluids wo n’t boil until they reach a temperature of 400/ to 450/F. But a light 2 % to 3 % contamination with water can drop that temperature by over 100/. Considering that most vehicles ‘ brake rotors have the friction-generating capacity to reach a radiance red-hot temperature in a long downhill application, this much boil point difference translates into poor brakes or no brakes at all at some orient along the descent. Brakes are designed to mechanically vent cooling tune through the rotors and wheels to shed as a lot of this heat as potential by convection. Brake pads often include a base corporeal chosen in function for its insulate capability, but there ‘s still enough heat to turn a hydraulic system into a pneumatic one, and therefore one that ca n’t stop the cable car .
The other major rationality for bleeding brakes, obviously adequate, occurs when a hydraulic component such as a hosiery, wheel cylinder or caliper is replaced or rebuilt. There is virtually no way to assemble the hydraulic lap on the car without introducing a bubble of vent into the connection, so you have to flush and bleed.

Bạn đang đọc: Bursting the Bubble

Why So Hard?
Why is it so much harder to bleed brakes on modern cars than on older models ? Because today ‘s hydraulic systems are so much more extensive. If you look at a conventional of the lines on a modern brake system-or the cable car itself for that matter-you ‘ll find well more steel lines and many more pressure-regulating components, all the more so if the car has such features as antilock brake or grip master. There used to be a metering/proportioning valve. immediately besides that there are accumulators, pumps, solenoids, valves and expansion chambers, all connected by lines. The more passages there are, the more places where an air travel bubble can be trapped vertically, and the more difficult it will be to remove .
This is compounded by the first rule of hydraulic systems : Bubbles want to float up. Since brake fluid normally flows downward when you bleed the system, the hang has to be fast enough and through the right tubes to carry the bubble along .
A alike trouble occurs sometimes with fastball cores. If person runs the hoses to and from the congress of racial equality backwards, the coolant can inactive flow at the normal rate, but there may be a big bubble trapped inside the congress of racial equality. While menstruate through the hoses and kernel is fast adequate, the coolant does n’t stay in the core long enough to shed its heat to the blown air. The symptom is little or no heat even with satisfactory menstruation, along with coolant in the render occupation that ‘s at about the lapp temperature as when it went in .
In a heater core, that ‘s lone inconvenient, except when it disables the defroster. In a brake system, a bubble in the incorrect position, fair like bracken fluent boil, can, in effect, mean no brakes at all. The bicycle falls to the floor, but the car continues to roll .
Power Bleeding & Vacuum Bleeding
power bleeders, which force brake fluid into the maestro cylinder and through the lines using shop air travel pressure as the source of energy, work very well for most brake systems. characteristic problems are much the result of poor proficiency rather than poor design : For model, if you leave the world power hemophiliac full of brake fluid, it can absorb water from the air fair sitting on the ledge. You could end up flushing the organization with brake fluid that ‘s worse than what ‘s in the car. besides, if you forget to vent the moisture from your compressor regularly, you could be using breeze at 100 % humidity to push that brake fluid. But when used correctly, a good power hemophiliac will let one person do a complete brake hydraulic flower very quickly .
Getting a bubble out, however, may be another narrative. When you pressurize the brake system, you pressurize the house of cards deoxyadenosine monophosphate well. And when it ‘s compressed, that bubble becomes smaller, distillery harder to flush from the system and still more likely to stick in place ascribable to local anesthetic open tension in the fluid. In such cases, a vacuum hemophiliac may do the job more successfully .
One thing to keep in mind before you work with most void bleeders : They ‘ll draw down the headmaster cylinder reservoir faster than you ‘re used to with two-man foot shed blood. This trouble does n’t occur, obviously, with reservoir-equipped power bleeders .
Before you start working with a vacuum hemophiliac, draw as much fluid as you can out of the chief cylinder ( to reduce the measure of old fluid you have to flush through ) and top it up with clean stuff. Due to the hygroscopicity of bracken fluid, water absorbed through, say, a back bicycle cylinder will disperse through the hydraulics in barely a matter of hours. But not sol fast as the fluid in your flower job, so you can count on a thorough fluid exchange angstrom soon as you see clean fluid at the far end of the pipe .
Be certain, however, that you constantly monitor the fluid degree in the passkey cylinder, because if it starts drawing air down the tubes, you ‘ll have a a lot hard problem than the one you started with. If you pull vent into the antilock hydraulic unit of measurement, you could have a real problem if a bubble gets behind a solenoid valve-a problem you may have to get solved at the dealer, since some carmakers distillery refuse ( presumably illegally ) to provide the information or sell the tools to cycle the ABS circuits. A utilitarian proficiency is to catch the cast-off fluid in a container so you have an accurate bill of how much you ‘ve removed ; it helps with the cleaning, besides .
Vacuum run, as mentioned, has the advantage that it stretches the bubbles, making them larger and more likely to exit the system. But there ‘s a problem characteristic of most brake chief cylinders : You must set the brake pedal down about one column inch to move the dominate cylinder pistons far enough to uncover the vents and allow a thorough sluice of the fluid behind the basal piston. Pedal depressors are simple and cheap tools that every shop should have ; they ‘re besides easy to fabricate if you ca n’t get one.

A promote difficulty with vacuum shed blood is that the threads around the hemophiliac screw are not airtight. The vacuum, therefore, has a inclination to draw air out down the threads and into the evacuate current, so ( if you use a crystalline tube that allows you to see the fluid ) it may look like there are still bubbles coming, even though there are none left in the system. so do n’t keep drawing fluid through constantly. A silicone-based ribbon sealant is gluey enough to seal most such gaps satisfactorily, and the sealant wo n’t react with the fluid to do something unexpected late. What ‘s more, the sealer will displace brake fluid left on the threads, which will otherwise absorb water and fix the hemophiliac in place-sort of nature ‘s own threadlocker .
Forcing the Issue
One of the most common pad-hanging bad habits is forcing the caliper piston back in with a C-clamp or a Godzilla-sized pair of Channellock pliers. The trouble is the second base rule of hydraulic systems : Debris and particulates want to sink and collect in the caliper digest at the lowest point in the system. When you force the piston back, you squirt a big amount of fluent at relatively high pressure binding through the hoses and lines, through the ABS hydraulic unit and to the master cylinder. This fluid can carry those bottom-dwellers along for the trip. While it wo n’t always happen, this drill could lodge a act of soil in a solenoid valve or a proportion valve, causing a atmospheric pressure leak late, when the valve is supposed to open or close .
The proper way to return the pistons into their bores, the brake guru agree, is by opening the hemophiliac, clamping off the hose and venting the fluid into a waste container while you squeeze the piston spinal column. While I do n’t expect discount brake chains to touch a hemophiliac if they do n’t have to, an freelancer shop class or a franchise with claims to superior craft should. This besides, of path, removes the dirty and most contaminated fluent from the locate where it could do the most injury .
Strange Cases
On this period, though, you may find the watch strange font : sometimes a brake hose starts to delaminate on the inside, allowing a rubberize flap to function about like a see valve. About every ten years ( if my have is a guidepost ) you ‘ll find a brake that will apply normally but wo n’t release right away because of good such an anomaly. normally, of course, brake hoses start to fail and crack from the outside, but be mindful that sometimes they fail wrong-side-out, and this is about the only symptom you ‘ll find .
Another rare but serious encase occurs when person has added the wrong fluid to the passkey cylinder-typically exponent steer or transmittance fluent. normally this will mean the seals on the headmaster will distort from the incompatible oil. Such a cable car is very dangerous because the rubber of the master cylinder piston skirts has had the same distorting chemical treatment, even though you ca n’t see it. The entirely specify for this one is to replace everything rubber eraser in the system. If the customer balks at the job, charge nothing and write Do not drive this car: No brakes! on the repair order. You might want to tow the car to wherever the customer wants, besides. Remember, brake and steer problems are possibly the most significant places for a denounce to drill legal self-defense .
But sometimes, none of these methods works. You still have a soft pedal point revealing a bubble somewhere in the system. Certain vacuum bleeders, like those made by RTI and Auto Shop-Aid, include a provision to constantly replenish the overcome cylinder with fluid during bleeding, so they can achieve a substantial vacuum-fluid menstruation without much risk of sucking air into the chief. These tools can besides come in handy if you have to draw out surfeit engine or transmission anoint after an accidental overfill .
There ‘s another possible fix-flushing the system backwards. When I first read about this proficiency, I was very doubting, supposing it would push debris upstream, excessively, and I ‘m still inclined to wait until I hear unsponsored reports. The fib, however, is plausible, and the technique worked when I tried it .
Phoenix Systems makes a new type of professional brake-bleeding tool that can do both vacuum and coerce bleed. It consists of a fluid cylinder with piston, connected to inlet and outlet lines. There are connectors for barely about every brake hemophiliac blueprint in the world, ampere well as adapters for most brake fluid cans and bottles and respective other accessories. The hemophiliac is well-made and does what they say. It will function as a vacuum bleeder similarly to vacuum and pneumatic types, but the pistol-grip natural process makes it easier to control the amount of fluid drawn out. I do n’t think it will draw quite the volume a power arrangement will, but that depends on how promptly you squeeze the manage .
The challenging claim they make is that, in very hard cases, once you ‘ve already flushed the organization so there ‘s nothing but clean fluid along with the troublesome ripple somewhere, you can reverse-flush the brake hydraulics, forcing fluid from the caliper or bicycle cylinder back to the dominate cylinder, carrying the bubble upward. Because of the abject volume and relatively low coerce, Phoenix Systems says, the chances of flushing backbone into a medium ABS hydraulic whole and other components is acceptably moo. precisely make indisputable you ‘ve flushed the system adenine completely as possible first. The early operation possible with the overrule proficiency is to pulse the trigger with a rapid series of short strokes, to dislodge any bubbles held in some inaccessible topographic point by airfoil latent hostility in the fluid .
This instrument is the best one I know of for bleeding a clutch bag overcome and slave cylinder arrangement-a system that much uses larger-diameter hydraulic lines than the brake arrangement and therefore is sometimes harder to push atmosphere down through. They include a special probe for slave cylinders with no run tap. Clutch hydraulics, of course, do n’t encounter the extreme temperatures of brake systems, but they ‘re just as capable to water concentration and corrosion of the home metals, so a regular flush at the lapp time as the brake fluid is fair care.

Most shops think small about spending several thousand dollars for electronic diagnostic tools, because with modern cars there ‘s good no alternative to getting this information. But the next time two of you are working on a car giving it the ol ‘ “Pump it up! Hold! Release” everyday over and over, figure out how much it ‘s costing you in shop fourth dimension to do it that way. Could be that one of these modern bleed tools is barely the way to save that time .
Download PDF

source : https://tonupboys.com
Category : Car Brakes