BY VIC LUCARIELLO
Hello, Bimmerphiles! This time out we are finishing up our three-part discussion of brake bleeding and brake-fluid flushing.
Last time out in Philes’ Forum, in the second installment of this series, we covered vacuum-
bleeding/flushing of hydraulic brake systems along with two methods of pressure bleeding/
flushing: gravity and that ol’ standby, Pump-The-Pedal. In the first installment, we talked about the difference between brake bleeding and brake-fluid flushing and the objectives of each
procedure. We left external-pressure bleeding/flushing for this installment.
There are two main types of external-pressure bleeding/flushing: conventional and “reverse”. In
conventional external-pressure bleeding/flushing, brake fluid under pressure is supplied to the
master-cylinder reservoir. This is typically accomplished with a specialty device such as supplied by Motive Products (see Photo #1), which has a special adapter that connects the master-cylinder reservoir in place of its regular cap. K-D Tools and Mityvac offer alternatives.
Before we go any further with external-pressure bleeding/flushing, let me strongly suggest that you use a reservoir-cap adapter specifically designed for BMWs (or whichever car you are working on). I have had very poor results using so-called “universal” adapters.
What I mean by “poor results” is that I got more brake fluid on the floor than I did into the brake system! Motive Products’ “European Bleeder 0100”, intended for the DIY set as well as
repair shops, comes with a specific adapter for BMW brake-fluid reservoirs. The Power Probe Company, in addition to making a high-end brake bleeder/flusher, makes some really trick (they really are works of art) aluminum adapters that they sell separately. I have specific Power Probe adapters for all our vehicles, including our Dodge pickup.
Motive Products’ bleeder/flusher, along with others of this type, resembles a hand-pump-type garden sprayer. After filling the Motive with the brake fluid of your choice, you connect it to your already-full master-cylinder reservoir and pump up the Motive to about 20 psi (pounds per square inch) pressure. Yes, there is a convenient pressure gauge right on the Motive tank. More
pressure than this can adversely affect the “plastic” master-cylinder reservoir.
Once the Motive is filled, connected, and pumped up, take your catch bottle and, after connecting it to the right rear caliper-bleeder screw with the appropriate tubing (I use clear, nylon tubing), crack open the bleeder screw and withdraw at least 250 milliliters (about a half-pint) of brake fluid before going on to the next bleeder screw.
On rear- and all-wheel-drive BMWs, I use the sequence of right rear, left rear, right front, left front. On front-drive BMWs such as the Mini, I use the sequence RR-LF-LR-RF. Can anyone tell me why the difference? When you are done withdrawing about a liter of fluid, double-check the amount of fluid remaining in the pressure tank, then do your clutch slave cylinder if your car has a manual transmission. Uh, I suspect that if you read Philes’ this question is irrelevant.
Detractors of external-pressure bleeding/flushing suggest that it is not good to have your new, fresh brake fluid in contact with pressurized atmospheric air (Remember: atmospheric air contains moisture and brake fluid is hygroscopic.). If this concerns you, there are higher-end brake bleeders/flushers that do not pressurize the fluid directly with air. These bleeders/flushers use either a diaphragm to separate the pressurizing air from the fluid or they (e.g., Power Probe) use a motor-driven pump to pressurize the fluid directly.
In my opinion, there is really no concern regarding pressurized air briefly contacting new brake fluid. In fact, in testing the boiling point of new fluid, and then immediately testing the same fluid after it has been installed via pressure bleeding, I see no change in the boiling point.
Other detractors maintain that some pressure bleeders are wasteful because there remains in the
bottom of the tank some brake fluid that should be discarded. If you are using one of the “boutique” (expen$ive) brake fluids, this can certainly be a consideration, depending upon the design of the bleeder/flusher you are using. I use a home-made, one-quart pressure bleeder that I pressurize with shop air. I designed the bleeder to use virtually all the brake fluid that is put in it, so wasted fluid is of no concern to me.
That brings us, finally (whew!) to “reverse” bleeding, which I have only used as a last resort when all the aforementioned methods have failed to remove air trapped in the brake or clutch (usually it’s the clutch) hydraulics. In reverse bleeding, pressurized CLEAN brake fluid is pumped into the calipers via the bleeder screws, and the fluid flows in the reverse direction, from the calipers (or clutch-slave cylinder) back up to the master cylinder, thence out of the top of the brake-fluid reservoir (AND onto the floor if you don’t remember to remove sufficient fluid from the reservoir!). A Phoenix Injector hand pump works very well for this procedure. Photo #2 depicts the Phoenix set up for reverse bleeding. The CLEAN bottle on the left contains fresh brake fluid, which the Phoenix will pump into the brake caliper (not shown). If the Phoenix has been previously used for vacuum bleeding, be sure to pump some clean brake fluid through it to ensure the unit and its connecting hoses are purged of old fluid and any air.
Although reverse bleeding can sometimes dislodge air that has resisted all other efforts, a danger of reverse bleeding is that any crud in the calipers or brake lines will be pushed back up, through the ABS/DSC (Antilock Braking System/Dynamic Stability Control) hydraulic unit, into the master cylinder along with the air bubbles you are trying so desperately to remove. So Rule One of reverse bleeding is to ensure that as much as possible of said crud has been removed from the system via conventional bleeding/flushing prior to trying reverse bleeding. Rule Two of reverse bleeding is to ensure the Phoenix and its connecting tubing are completely purged of air prior to being connecting to a caliper.
Speaking of ABS/DSC, inveterate Philes’ Forum correspondent Art Neufeld posed the following question after reading the first two parts of this series: “I used to see strong recommendations not to self-bleed cars with (ABS/DSC). Are you going to discuss this, and how to do it effectively?”
The ABS/DSC hydraulic (ABS) unit is generally plumbed into the brake system between the master cylinder and brake calipers. So any air that is in either the master cylinder (perhaps because it was changed) or the lines connecting the master to the ABS will have to pass through the ABS in order to be purged out through the calipers if conventional bleeding is being used. Generally speaking, this is not a problem. When the ignition is off, the ABS internals are isolated from the rest of the brake hydraulics, so an air bubble can pretty much pass through the ABS without getting stuck there. Of course, the sometimes serpentine brake-tubing arrangement surrounding the ABS can tend to trap air bubbles, but the ABS itself is not usually the problem.
I think that some folks have gotten into trouble when bleeding ABS-equipped cars by turning on the ignition while the bleeding is being done. This can cause the ABS to ingest some air. Once this occurs, it may require a professional-level scan tool to operate the ABS and expel the air.
In the majority of cases where the brakes need bleeding, it is because a caliper or hose has been changed. So, provided that the system is not allowed to drain out while the caliper/hose is being replaced, air upstream of the ABS is not an issue. In the case of a fluid flush there should be no air in the system to worry about.
So, Bimmerphiles, that brings us to the conclusion of this series on brake bleeding and brake-fluid flushing. I hope you have found it to be informative. If you have any questions or comments, please do not hesitate to email me.
Anyone wishing to contribute to Philes’ Forum can contact me at [email protected] 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. .