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Experience with DOT 5 (Silicone) Brake Fluid

Do any of you have personal experience with DOT 5 brake fluid in Beemers? If so, I’d appreciate hearing from you, as to whether it was good or bad.  In the meantime, I’ll share mine with you.

My bike is a 1974 R90/6, purchased in December 1976 with 5k on the odometer.  It has a single-caliper front disc brake.  Removal of the fuel tank to top-up the brake fluid four months later revealed a jet-black liquid in the brake system, which I flushed out with clean DOT 3. Occasional checks thereafter revealed that the fluid in the reservoir remained clean and clear, and there were no leaks in the system.  By  1981 (31k), the fluid in the reservoir was starting to look like Sierra Nevada pale ale, so I flushed the system again with DOT 3.

In 1982, having had good luck with silicone (DOT 5) brake fluid in a 1971 MGB and a 1966 Buick for about three years, I decided to try it in my bike.  Advice to the contrary notwithstanding, I disassembled the master cylinder and front caliper, found the pistons and cavities clean and bright – no rust or corrosion – wiped out all the old fluid with a clean rag, washed loose parts with soap and water, reassembled using the original seals, and filled the system with DOT 5.  There had been a very slight seep between the master cylinder and the reservoir, but decided that it wasn’t bad enough to go to the trouble of removing the reservoir.

1990 (45k):  Added brake fluid to top-up the reservoir.

1993 (48k): Bled brake.  Still seeping a little at the reservoir joint, but not bad.

1997 (52k):  The front brake was still working O.K. with no apparent leaks or sponginess of the brake lever, but the seepage at the reservoir/master-cylinder joint seemed to be a little worse, and removal of the reservoir revealed a flattened o-ring and some rust around the sealing surface.  Precautionary removal of the master cylinder piston revealed that all piston seals were still in “as-new” condition. However, there was rust in the bore near the entry hole from the reservoir, most likely caused by my failure to

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