Philes’ Forum – Summer 2020
By Vic Lucariello, Sr
Last time we talked about brake-system bleeding and brake-fluid flushing and the purpose of each. Bleeding is intended to remove any air or other gas bubbles from the hydraulic system, while flushing is done to replace old, contaminated brake fluid with fresh new fluid. Of course, a good flush will tend to remove any entrained gasses. Air or gas bubbles in your brake [or clutch] system can cause a low, “spongy” pedal, while contaminated fluid, in addition to fomenting corrosion, can boil under severe-use conditions and cause……………a low, “spongy” brake pedal. Generally speaking, when all is said and done, the main difference between brake
bleeding and brake-fluid flushing is the amount of fluid put through the system.
There are several methods of bleeding brakes and changing brake fluid, and some methods may be better than others for problem situations. With one
exception, all the methods we will talk about involve fluid movement from the master cylinder, down to the calipers and out of the system via the bleeder screws. In the case of brake bleeding, the idea is that any air will be expelled with the discharged fluid. I guess the various methods can be categorized as “pressure” or “vacuum”. Let’s begin with vacuum.
Before we begin, note that regardless of what method you use, you should be capturing all expelled brake fluid in a suitable container via a piece of
tubing attached to the bleeder screw. I always use clear-vinyl tubing so that I can observe the color of the expelled fluid as well as any bubbles. Suitable vinyl tubing can be had at any hardware store. Brake fluid handily removes most paints. And, trust me on this, you definitely do not want to get brake fluid in your eyes! So, eye protection is required, as it is for just about any work on your car.
Vacuum bleeding/flushing involves applying suction to the caliper-bleeder screws. This can be accomplished with a Vacula or Mityvac shop-air powered “brake bleeder”, or with a simple hand pump as shown in Photo #1. This particular hand pump is by Phoenix Systems, and it is suitable for both “normal” and “reverse” bleeding [more on this later]. While the air-powered vacuum bleeders are faster, the hand pump works just as well.
Although vacuum bleeding/flushing is popular with many folks and is relatively fast to set up, there are a couple of disadvantages to it in my opinion. I guess what bothers me most is that, being that suction is applied to the bleeder screw, you can get fugitive air sucked in around the bleeder-screw threads, and you can’t tell if this air is coming from the brake system or sneaking around the bleeder screw. You can minimize the amount of fugitive air by wrapping the bleeder-screw threads in Teflon tape. However,
be SURE to keep the tape off of the tapered seat on the bleeder screw. Photo #2 depicts a bleeder screw wrapped in Teflon tape. Teflon tape comes in different qualities and thicknesses. The thin, good quality stuff is what I prefer.
A purported advantage of the vacuum method is that it tends to enlarge any bubbles in the system, thereby making them easier to entrain and remove. This sounds quite reasonable to me.
Vacuum bleeding/flushing is generally a bit slower than pressure bleeding [more on this later], and usually vacuum bleeding can only be applied to one bleeder screw at a time. Moreover, one needs to keep close watch on the brake-fluid-reservoir level [this applies to some other methods as well] to ensure that it does not empty and introduce air into the brake system.
Pressure bleeding/flushing can be sub-divided into several categories: gravity, pump-the-pedal [P-T-P], and external pressure. In the gravity method, one simply opens one or more bleeder screws and allows fluid to flow from the system. The gravity method is perhaps the slowest of all the methods I know of, and in some cases, depending upon the arrangement of the system and how long the brake lines are, one may get little or no brake-fluid flow from one or more bleeder screws, especially the rears. Also, being that this method is relatively slow, one may tend to get impatient and walk away, perhaps forgetting to keep an eye on the all-important brake -fluid-reservoir level. I once did a survey of professional shops specializing in BMWs and some shops claimed to use gravity bleeding/flushing.
That brings us to pumping the brake pedal. In this very popular [in the DIY set] method, one has an assistant pump the brake pedal, then hold foot pressure on it while the bleeder screws are opened one at a time. The pumping action of the master cylinder is used to expel fluid and any entrained gas. Then [hopefully] after the bleeder screw is re- closed, the assistant releases the brake pedal and the sequence is repeated…..over….and…… over……..and, well, you get the picture. Any of you who has spent any time in repair shops or track garages has undoubtedly heard the “Pump it up…..Hoooold it….OK” litany. One of my first jobs when I began working in the corner “gas station” lo those decades ago was to be the P-T-P assistant. And that reminds me of one of the disadvantages of the P-T-P method.
When doing the ol’ P-T-P routine, your assistant has to be very careful not to release the brake pedal before you say “OK”. [Of course, you need to be very careful not to say “OK” until you have closed the dang bleeder screw.] If the pedal is released before the bleeder screw is closed, the system will suck in a nice shot of air. In this august, family- oriented publication, I cannot repeat what ol’ ‘Pino Cocuzzo said to me in that Gulf station the first time I took my foot off the brake pedal too soon.
Speaking of less-than-competent assistants, I’ll never forget the time I was bleeding the brakes on my hotrod in an effort to alleviate a spongy brake pedal. I must have repeated the “Pump it – hold it” litany for 15 minutes [at least it seemed that long] with no success before I realized that my assistant was depressing the CLUTCH pedal [no mean feat on my hotrod!].
Assistant incompetence aside, my main concerns regarding P-T-P bleeding/flushing are that it takes quite a while, and many pedal cycles, to pump a liter of fluid through the system, and that the master-cylinder’s piston seals are dragged repeatedly over areas in the master-cylinder bore that they normally do not contact. In uncommon cases [perhaps more likely with older, cast-iron-body master cylinders], this can cause the master cylinder to fail. Yes, this has happened to me. Of course, one needs to keep close watch on the fluid level when using the P-T-P method. And a closer watch on the assistant!
The P-T-P method does have one great advantage over the other methods we are talking about. Even moderate foot pressure on a brake pedal can produce 1000 psi [pounds per square inch] pressure in the brake system. To put that in perspective, the pressure provided by a common pressure bleeder [more on this later] is only about 20 psi. Opening a bleeder screw with 1000 psi behind it results in a high-velocity jet of brake fluid, and this high velocity can sometimes expel a recalcitrant air bubble that has resisted other methods of brake bleeding. I rarely have to resort to P-T-P when bleeding a brake system. And of course, when you are doing a simple fluid flush, there should be no air in the system to begin with.
That brings us to external-pressure bleeding/flushing and unfortunately to the end of Philes’ Forum for this time. See you next time, Bimmerphiles. Anyone wishing to contribute to Philes’ Forum can contact me at firstname.lastname@example.org. I’m interested in tech tips, repair/maintenance questions, repair horror stories, emissions-inspection sagas, product evaluations, etc.