Experience with DOT 5 (Silicone) Brake Fluid

Part 1, written by J Baslo, Snowbum’s reply is Part 2.

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 remove all traces of DOT 3 from the reservoir/master-cylinder joint 15 years earlier.  Fortunately, I was able to clean it up, install a repair kit (which included a new o-ring seal between the reservoir and master cylinder), and put it back together.

2006 (72k):   I’m not an aggressive rider, but recently learned from David Hough the advantages of using a little front brake while canyon carving, so I’m using the front brake a little more now, and have decided to replace the original front pads with new ones. Fuel-tank removal revealed all O.K. with the master cylinder, and the caliper piston moved freely with no leaks, but I decided to check the inside of the caliper in light of the rusty-master-cylinder discovery nine years ago.  Removal of the caliper piston (38 mm dia.) revealed no rust or corrosion in the bore, a piston seal in “as-new” condition, and a dust seal in good condition except for a couple of small tears possibly caused by over-stretching during removal.  The chrome-plated piston surface had some light rust on both sides of the piston seal.  My guess here is that either the piston seal had been torn for a while, or residual traces of DOT 3 on the walls of the brake hose gradually migrated down to the caliper where it remained entrapped.   Either way, those spots were easily cleaned up with a very fine stone, and I would have re-installed the original seals had it not been for the small tears in the dust seal.  Inspection of the original brake hose between the master cylinder and the caliper revealed no cracks and, prior to disassembly, there were no perceptible bulges in the hose when the brake lever was squeezed.

In summary, after 24-years continuous DOT-5 usage in my 1974 R90/6 and infrequent maintenance of the hydraulic brake system, there has been no deterioration of the elastomeric parts, and the only problems appear to have been self-inflicted by my failure to remove all traces of hygroscopic DOT 3 fluid.

If any of you have similar/contrary experience, I’d appreciate hearing from you.
My e-mail address is jbaslo@sbcglobal.net.
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Part 2; COMMENTS by Snowbum:

DOT 5 silicone fluid is not compatible with most Brembo rubber components. DOT 5 silicone fluid foams, is compressible a bit!!, water vapor tends to condense inside and create havoc over time after settling into droplets.  TWO of the several possibilities for troubles include sudden freezing up of the brakes, AND, spongy brakes with possible total loss of braking, due to the microscopic foaming of silicone fluids.  DOT 5.1, a higher temperature rated version of DOT 4, often sold as Racing brake fluid, is compatible but I recommend against it, as it deteriorates quickly and needs 6 months to 1 year maximum replacement.
SOME rubber parts in ATE calipers, such as on the above author’s motorcycle are probably compatible, to some extent, with silicone DOT5.   How can you tell?  You possibly can’t until damage is done.
DOT 3 and DOT 4 are safe, easy to bleed and change yearly, work with all BMW braking systems and all replacement rubber parts and original parts.
I am absolutely against using DOT 5 silicone fluids in your BMW Airhead motorcycle brakes.  YOU HAVE BEEN WARNED!

Snowbum
ABC 1843