Philes’ Forum – Spring 2020


By Vic Lucariello, Sr.

Hello, Bimmerphiles! Pursuant to a number of recent inquiries, this time out I am going to talk about brake fluid flushing and brake bleeding. What, you say, they are the same thing? Ahhh, read on…..

The hydraulic brakes found on any modern passenger car depend upon a principle of hydrostatics that pretty much states that the pressure in a hydraulic system under static [non-flowing] conditions is the same throughout the system [given no elevation changes]. So when you step on your brake pedal and pressurize the brake fluid in the brake master cylinder to, say, 1000 psi [pounds per square inch], this same 1000 psi pressure is applied equally to each of the brake calipers via small pipes that connect the master cylinder to the calipers via the ABS module, thereby applying the 4 brakes. This might sound pretty simple, but the adoption of hydraulic brakes in the 1920s was one of the most significant advances in the development of motor vehicles. Some manufacturers, like Ford, resisted the adoption of hydraulic brakes, continuing to rely on mechanical brakes with their system of levers and linkages under the car.

An important factor in the performance of hydraulic brakes is the incompressibility of the brake fluid between the master cylinder and calipers. Most liquids are virtually incompressible, at least at room temperature, while gasses, such as air or steam, are quite compressible. Hence if you have any gas bubbles in your brake system, the result will be a “spongy”, or soft, low brake pedal. Or in an extreme case, a brake pedal that goes to the floor. Depending upon where in the system the gas is, it can also cause the vehicle to veer right or left while the brakes are being applied.\

How does air get into a brake system? While in rare cases, air can sneak in through a bad master-cylinder or caliperpiston seal, air mostly enters systems when a component is disconnected for repair or replacement. Regardless of how air has entered a system, the procedure known as “bleeding the brakes” is intended to remove said air.

As a side note, brake calipers are in many cases physically interchangeable side-to-side. However, if a caliper is installed on the wrong side, usually the bleeder screw is in the wrong location. This makes bleeding the brakes either extremely difficult or impossible. I have read reports that professional technicians have made this mistake.

The polyglycol-based DOT 3, DOT 4 and DOT 5.1 brake fluids found in virtually all cars today are hygroscopic, meaning that they have an affinity for and tend to absorb moisture. A typical DOT 4-rated brake fluid still in the can might have a boiling point of about 500 degrees Fahrenheit, while water of course boils at 212 degrees Fahrenheit. As you might expect, a mixture of the two will have a boiling point below 500 degrees Fahrenheit. According to a graph in Brake Handbook, by Fred Puhn [HP Books, 1985], a typical brake fluid will boil at only about 350 degrees Fahrenheit after it has been contaminated by only 1% water. According to another graph in this same book, this 100 degrees Fahrenheit reduction in boiling point will occur before the brake fluid has been in service for 6 months. My own brake fluid-boiling-point data, collected for more than a decade now, suggests that a 100 degrees Fahrenheit drop in boiling point in only 6 months would be quite unusual. In fact, I have never seen new fluid degrade that much in 6 months.

How does this moisture get into the brake fluid? Mainly through the vent in the master-cylinder-reservoir cap. [Some cars have a “rubber” diaphragm under the cap to minimize contact of the brake fluid with air.] Some say that a lesser amount gains entry through the brake hoses via osmosis. Regardless of its point of entry or method of entry, moisture does infuse the brake fluid, and this is a bad thing.

Although water in the brake fluid foments corrosion of ferrous components in the brake system, for performance driving [or driving in hilly terrain] the boiling-point suppression is by far the more diabolic villain. When the brakes get hot enough to exceed the boiling point of the brake fluid, gas pockets begin to form in the calipers and brake lines. Remember: gas is compressible. Although you might get a warning in the form of a “spongy” brake pedal, in some cases the driver’s first inkling that something is wrong is that the brake pedal goes to the floor! Then, assuming you don’t crash in the interim, after the brakes cool and the gasses condense back into liquid, the brake pedal is magically restored – until the next time the brakes get hot…………

Obviously, the only way to keep your brake fluid at or near its rated boiling point is to flush out frequently the old fluid and replace it with new, quality fluid from a sealed container.

So there you have the difference: Brake bleeding is intended to remove entrained air from the brake system while brake fluid flushing is intended to replace contaminated fluid with new fluid. In many cases, such as the replacement of a caliper or brake hose, the brake bleeding procedure only involves expelling a few CCs of fluid from one caliper – just enough to get the air out. Proper fluid flushing, on the other hand, will require putting at least a liter of new fluid through the system; and of course opening up all the bleeder screws. So, as part of your driver-school-car-prep regimen or normal brake maintenance, you need to flush out the brake fluid,not merely “bleed the brakes”. Some shops, if you bring in the car and ask them to “bleed the brakes”, will do just that: bleed the brakes. The fact that you are not getting a fluid flush won’t be their fault, either.

If you have a shop do your brake fluid flushing, I recommend that you bring the brake fluid of your choice in an unopened 1-liter container and tell them you want the entire contents put through the system. A labor charge of 45 minutes to an hour is appropriate.

What methods are available for flushing fluid and bleeding brakes? I’m glad you asked. More on this next time!

Anyone wishing to contribute to Philes’ Forum can contact me at I’m interested in tech tips, repair /maintenance questions, repair horror stories, emissions-inspection sagas, product evaluations, etc.

Copyright 2020; V.M. Lucariello, P.E.

Philes’ Forum – Summer 2017


By Vic Lucariello, Sr.

Hello bimmerphiles! In this sojourn into the Philes’ Forum chronicles, I have a follow up to my last column together with a new item on brake fluid.

Last time out, in the Spring 2017 Bulletin, I wrote about using Dexron III-type automatic-transmission fluid [ATF] vs Pentosin CHF 11.S fluid in your Bimmer’s power steering. [You should be able to download the Spring 2017 Bulletin from the NJ Chapter Website.] I advised that, if your Bimmer uses Dexron fluid, you should not change the system over to CH 11.S or mix the fluids. Since I wrote that, additional information has been forthcoming.

After reading the Spring 2017 Philes’, bimmerphile Sal Puleio, inveterate owner of Rennsport Motor Works in Hackensack, contacted me and sent a copy of a BMW service information bulletin, or SIB. [In the auto-repair industry, these bulletins are generically known as TSBs, but BMW has their own name for them.] The SIB in question does corroborate what I had previously written [and what was confirmed by Matt Kimple at Bridgewater BMW], “The mixing of CHF and……ATF is NOT permitted [emphasis mine]”.

However, the SIB goes on to state that if a power-steering system is to be converted from Dexron III to Pentosin CH 11.S, “….the system must be drained as completely as possible.”

Sal and I discussed this, and we concur in recommending that if a power-steering system is to receive a fluid conversion, simply draining the system “completely” is not the best way to go. We recommend that the system be drained [hoses disconnected] then filled with the new fluid [Ah…hoses reconnected], then the vehicle started and warmed up, with the steering moved from lock-tolock [fully right, then fully left] a number of times. Then drain the system again and refill with the new fluid after changing the fluid reservoir.

Why would you want to change your steering-fluid type? Glad you asked. In specific cases, such as certain E46s [3- Series in production from 1998 – 2006] with powersteering noise under certain operating conditions, it may be beneficial to convert from the original Dexron III to Pentosin CH 11.S. According to several sources, CH 11.S has about half the viscosity [measured at 40-degrees C (about 104F)] of Dexron III. Viscosity can be roughly defined as resistance to flow. However, converting an old system to a less-viscous fluid may foment leakage, so be advised of this as well. If you effect such a conversion, the new reservoir will probably already be marked indicating that CH 11.S should be used. If it is not marked, be sure to source or make a label for it.

Much thanks to Sal for taking the time to look up and send me the SIB and for consulting with me on this topic. Rennsport is located on Berlews Court in Hackensack [201- 489-5577]. Sal has been taking care of NJ-Chaptermember Bimmers since 1981, and he has professionalautomobile- technician experience preceding that. Legend has it that Sal advised Henry Ford on the type of transmission to use in the Model T [or was that Henry Ford II and the Edsel?], but that, folks, was before my time.

While we are on the subject of fluids, here is an update on my continued testing of brake-fluid boiling points. In commissioning my new shop here in CO, I finally unearthed the Chapter’s brake-fluid tester.

For those who do not remember my previous articles on the subject, the DOT 3 and DOT 4 brake fluids found in most vehicles today [including the DOT 4 low-viscosity fluid in recent Bimmers] are hygroscopic, meaning that they have an affinity for moisture. Atmospheric moisture gains access to the brake fluid via the master-cylinder-cap vent, the seals on the caliper pistons, and, some say, via osmosis through the brake hoses. The effect of this moisture is twofold: it foments corrosion of the brakesystem components and it reduces the boiling point of the brake fluid. Since brakes get hot in operation, if this heat causes the brake fluid to boil, partial or complete loss of braking will occur. BMW and some other manufacturers recommend periodic replacement of brake fluid, while some other manufacturers inexplicably do not.

It has been my conclusion, particularly after observing vehicles being driven by driving-illiterate folks negotiate the approximately 8-mile descent of Wolf Creek Pass [they drag their brakes almost continuously as opposed to using them as briefly as possible and then letting them cool between applications] that it is pretty hard to boil your brake fluid on the street, even if you try like these folks are doing. [Of course it is a different story on the track.]

An example of this hit [too] close to home when my daughter visited last month. [Thankfully, she does know how to use her brakes properly.] While she was here, I renewed the front-disc-brake pads in her ’01 Jeep Cherokee, and in doing so took brake-fluid samples from both front calipers. The boiling point of this unknown-age fluid tested at only 370 F! Typical parts-store DOT 3 brake fluid tests at maybe 500 F out of the can, while premium DOT 4 fluids like Ate Type 200 test at around 570 F. Some of the boutique [read: expensive] fluids purport boiling points of 600 F or more, but that is fodder for a future Philes’ Forum. The Cherokee has negotiated Wolf Creek Pass a number of times with this brake fluid [see Photo #1], so my conclusion seems justified. After checking Amy’s brake fluid, I asked her how her brakes had felt coming through Wolf Creek Pass. She said they were fine.

Photo #1 – Can This Be MY Daughter’s Brake Fluid?

That’s all for now, bimmerphiles. See you next time. Anyone wishing to contribute to Philes’ Forum can contact me at I’m interested in tech tips, repair /maintenance questions, repair horror stories, emissions-inspection sagas, product evaluations, etc.

© 2017; V.M. Lucariello, P.E.

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