Issue link: http://digital.nexsitepublishing.com/i/108533
June Tech noTes Porsche Brake MainTenance All AllAn CAldwell Disc braking systems have always been one of the best features on most Porsche models ever since they were first introduced on all 356C and 356SC production cars in 1964. One of the best things about Porsche disk brakes is their relative simplicity and ease of maintenance. A number of previous articles (References 1 and 2) have detailed the proper procedures and methods. However, since previous reviews, there have been a number of changes related to brake fluid that make the subject worthwhile to review. For do‑it‑yourselfers, a convenient time to do a brake check is while the car is up on jack stands for its regular service. The major brake maintenance‑areas include: • Periodic replacement of brake fluid and system bleeding. • Monitoring of pad wear and replacement of pads when needed. • Inspection of rotor condition and wear. • Reconditioning or replacement of master cylinders, calipers and flexible brake lines when needed. The most significant maintenance item in the above list is brake fluid replacement and bleeding which needs to be addressed nor‑ mally at two year intervals and possibly every year depending upon the severity of use and the type of brake fluid used. If the Porsche has brake pad wear sensors, it is also important to periodically check brake pad wear and install new pads before the wear sensor wires wear out, which often results in having to replace not only the pad but the sensors as well. And once the Porsche has 12 or more years service, the brake hoses can deteriorate from the inside out due to chemicals in the brake fluid which cause the internal fluid channel to shrink to too small an opening and result in the brake pressure not releasing (lots of brake dust suddenly starts appearing on the front wheels). All of these issues have been addressed in past tech articles listed in the references to this article. The hygroscopic nature of most brake fluids in picking up mois‑ ture is the key driver in the recommendation for frequent replace‑ ment (silicone fluids are an exception, but are not recommended for general use). Even small quantities of absorbed moisture, on the order of one percent, can reduce the boiling point of brake fluid by 50F. Conventional fluids will absorb 1.5 percent moisture or more from atmospheric humidity in the course of one year. Because a single stop from 70 mph can result in a brake temperature rise of 100F, repeated heavy braking and temperature build‑up can result in the fluid boiling if it has absorbed very much moisture and lowered the boiling point. Therefore, it is important to maintain the highest possible brake fluid boiling point. Brake fluids sold in the U.S. carry Brake fluid 36 June 2011 a DOT (U.S. Government Department of Transportation) rating that indicates the ability of the fluid to maintain a high boiling point both with no moisture absorbed (referred to as "dry" conditions) and with 3.5 percent moisture absorbed ("wet" conditions). Figure 1 shows the DOT specifications for brake fluid minimum boiling points for the three common ratings. Note that the DOT specifications are the minimum values and are often exceeded in varying amounts by the available fluids. DOT SPECIFICATION 3 4 5 MINIMUM BOILING POINT, DEGREES F DRY (0% MOISTURE) 401 446 500 WET (3.5% MOISTURE) 284 311 356 The original ATE brake fluid that came in Porsches from the factory until the early 1980s was a conventional DOT 3 fluid. Start‑ ing in June 1984, all Porsche production cars came with DOT 4 fluid, and the factory recommended that all previous cars also use it (Reference 3). Up until the early 1990s, DOT 4 brake fluids, with a minimum wet boiling point of 311F, had the highest wet boiling point for conventional fluids and were the fluid of choice for most applications. The factory‑recommended change interval for general street use with conventional DOT 3 and 4 fluids was always two years, which is based on an average moisture absorption rate. Many owners feel that for maximum performance with conventional fluids, the only sure method for maintaining the least moisture in the brake fluid is yearly changing. Figure 1. DOT Brake Fluid Minimum Boiling Points. In the past, the DOT specification did not include the generic base compound used by a given type of fluid, such as "conventional" (glycol ether), silicone, or synthetic. However, in the past, the only fluids that could meet the DOT 5 minimum boiling point tempera‑ tures were silicone‑based fluids and many people came to regard DOT 5 as essentially a silicone specification. Although silicone brake fluid has no affinity for water and has some important applications in cars that are stored or not driven for long periods of time, it is incompatible with conventional fluids, is difficult to bleed and can have some strange pedal feel problems. Consequently, it has not been recommended for general road use (see Reference 4). In the meantime, however, the Europeans were busy developing synthetic oils and brake fluids and have come up with some syn‑