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At the 4WD Mechanix Magazine site, you will find my article and series of how-to videos on forming brake tube flares.  The information below is excerpted from that URL page.  To see the entire article and HD videos on how to form tubing flares, visit:  http://www.4wdmechanix.com/video-series-how-to-flare-automotive-brake-tube-fuel-lines-and-cooler-tubing/

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Brake rubber parts are vulnerable to damage, wear and deterioration.  One of the quickest ways to cause brake rubber parts failure is exposure to petroleum or mineral base solvents, oils or compounds—including the popular products shown at left.  The time honored and safe substance for cleaning brake parts is denatured alcohol.  Always dry parts thoroughly before assembly…Periodic brake fluid changes can be done with a vacuum bleeder at the wheel cylinders and calipers, using the correct and fresh brake fluid.

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Brake service work on cylinders and calipers will benefit from a castor base grease like Millers Red Rubber Grease.  Assembly of hydraulic cylinders and calipers can benefit from Raybestos BAF-12 Brake Cylinder Assembly Fluid.  Both of these products are harmless to brake rubber parts and recommended for the installation of caliper piston seals or brake wheel cylinder cups.

Rubber/Chemical Compatibility, Periodic Brake Fluid Changes and Choosing a Brake Fluid

Always use the brake fluid type recommended by the vehicle manufacturer.  DOT 3, DOT 4 and DOT 5.1 are compatible glycol-based brake fluids.  (Use the recommended type, DOT 3 and DOT 4 are often combined ratings.)  Never mix DOT 5 silicone brake fluid with DOT 3, 4 or 5.1.  The difference between DOT 3, 4 and 5.1 is the boiling point, typically the higher the number, the higher the boiling point.  (Some DOT 4 racing brake fluids would be an exception, they have a very high boiling point.)  DOT 5.1 and DOT 5 Silicone have boiling points nearly the same but these fluids are not interchangeable, and they are not compatible.  If your system has recommended DOT 3 or DOT 4, adding or changing to a DOT 5.1 glycol based fluid is acceptable.  DOT 5 Silicone brake fluid can only be used by itself in a system completely free of glycol-based brake fluid.

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At left is a contemporary Castrol brake fluid previously labeled DOT 4 GT/LMA.  The “LMA” stands for “Low Moisture Absorption”.  This LMA feature is desirable and allows consumers to use a DOT 4 glycol base fluid with more resistance to moisture.  (Moisture lowers the boiling point of brake fluid and causes system corrosion.)  DOT 5 Silicone Brake Fluid (shown at right) is hydrophopic—it will not absorb water.  Silicone brake fluid requires a system that is completely purged of all glycol based fluid and moisture before filling with DOT 5.  If there is previous moisture present, the DOT 5 silicone brake fluid will not absorb the moisture; instead, the moisture will form hazardous bubbles if the fluid reaches the moisture’s boiling temperature.  Never mix these fluid types.  For more details on DOT 5 Silicone brake fluid characteristics, visit the Clearco Products page at:   http://www.clearcoproducts.com/dot5-brake-fluid.html.

Warning:  Never use a mineral oil or petroleum product in a brake system designed for a castor oil/alcohol brake fluid (DOT 2), glycol-base brake fluid (DOT 3, 4 or 5.1) or a silicone type brake fluid (DOT 5).  Mineral and petroleum products can cause rubber to swell and fail…Gasoline or diesel fuel, WD40 or any other petroleum based product or solvent should never be used for cleaning or freeing up brake system parts that contain rubber.  Also note that brake parts cleaners often are intended for metal parts like brake backing plates and hardware—not for use within the cylinders or around any of the rubber seals!  Read labels carefully…There are isolated vehicle brake systems (exotic European models like some Citroen and Rolls-Royce cars) that specify the use of a special mineral oil brake fluid.  Always use the brake fluid type that the vehicle manufacturer recommends for a specific make, model and year vehicle.  On modern vehicles, the master cylinder cap often has an inscription that states the recommended brake fluid…If a product’s chemical compatibility with rubber is questionable, see the Mykin chart at: http://mykin.com/rubber-chemical-resistance-chart.

Identify the rubber type and recommended brake fluid type when working on brake systems.  Traditional brake seal and cup rubber is now being replaced by EPDM and other synthetic rubber materials.   Vintage vehicles with pre-DOT 3 fluid use a DOT 2 or equivalent brake fluid that is typically castor oil (vegetable base, not mineral) and alcohol.  This is why denatured alcohol is discussed in older shop manuals as a suitable “flushing” fluid for the brake system.   The castor oil will not harm rubber and neither will alcohol.  However, if denatured alcohol is used as a flushing agent, the lines and cylinders must be allowed to dry completely before replacing all rubber parts and replenishing the system with fresh brake fluid.  Drying can be sped up with the use of compressed air if the air is filtered and does not contain moisture.  Flushing with denatured alcohol is only done when all rubber seals are renewed after the flush and drying of the lines and cylinders.  If the cylinders are in good condition and rubber is known to be okay, leave them alone; simply exchange the brake fluid with fresh brake fluid.

Brake fluid changes get sorely neglected in the modern era, which means that corrosive moisture, a declining boiling point of fluid and brake fade can be an issue with hydraulic brake systems.  It is relatively simple to at least vacuum bleed the brake system periodically at the wheel cylinder or caliper bleeder valves.  Drawing fresh fluid through the system, from the master cylinder to the wheel cylinders or calipers, can at least remove the old hygroscopic (moisture absorbing) fluid and any debris.  Arguably, vacuum bleeding is more effective than power bleeding for purging contaminants from a hydraulic brake system.  Power bleeders apply pressure at the master cylinder, which can push debris to the edges of the wheel cylinders or calipers.  Vacuum bleeding draws old fluid through the bleeder valves and will suck out debris from within the cylinders or calipers.  If the cylinders remain assembled, vacuum bleeding will likely do a better job.

Footnote:  In the heyday of under-floorboard master cylinders, hydraulic brake system flushing annually was commonly recommended along with the replacement of brake cylinder rubber parts.  Vented master cylinders were notorious for absorbing atmospheric moisture, dirt and even road surface water.  DOT 2, DOT 3, DOT 4 and DOT 5.1 (not to be confused with DOT 5 Silicone) brake fluids are all hygroscopic:  They absorb water/moisture at the rate of at least 3% volume per year in the average climate and under normal conditions.  A passenger car or 4x4 vehicle with the master cylinder mounted to the vehicle's frame beneath the floorboard, like the vintage Jeep models and WWII to early Vietnam era military trucks, is highly susceptible to drawing moisture through the vented master cylinder cap, especially during stream fording or when the vehicle is stalled in body sill depth water!  Before the use of rubber bellows on master cylinder cover gaskets and other moisture barrier methods at the master cylinder cap, a periodic brake fluid change and hydraulic brake cylinder rubber parts replacement was considered an annual service task.  

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EIS master and wheel cylinder rebuilding kits were popular service parts in the vintage vehicle era.  Vehicle manufacturers recommended an annual brake system flush and rubber parts replacement.  See the vehicle manufacturer’s recommended fluid change intervals, usually one to two years on modern vehicles that use glycol-based (DOT 3, 4 or 5.1) brake fluid.  This kit (photo courtesy of a seller’s eBay ad) contains the typical service parts for an annual flush and rebuild of wheel cylinders on a 1930s to 1966 U.S. vehicle equipped with a vented-to-atmosphere, single circuit master cylinder.  Fluid quickly became moisture and debris contaminated, especially on earlier vehicles with the master cylinder exposed beneath the body!  Dusty roads and stream crossings were highly abusive to brake fluid, iron hydraulic cylinders and rubber parts.

For a better understanding of rubber types and their chemical sensitivity, see the Mykin Chemical website’s “Rubber Chemical Resistance” chart:  http://mykin.com/rubber-chemical-resistance-chart.  To better understand modern brake fluid characteristics and specifications, see this well written piece by Steve Ruiz at the Centric/StopTech site:   http://www.stoptech.com/technical-support/technical-white-papers/brake-fluid

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