BMW R1200R R1150R/T F800 K1200R R1100R/T R1250R RnineT Message Board

Hello friends,
I noticed that the rear brake fluid reservoir was completely empty, so I filled it up. Strangely, the fluid did not drain down into the tube below. I thought it must be air bubbles in the system, so I tried bleeding the rear brake. I got some fluid out, but the fluid in the reservoir wouldn't move. I pumped and pumped to get that fluid to drain out of the reservoir, with no luck. I decided to check the reservoir for blockages, and removed a seal or inside lining of the reservoir that the fluid was sitting in. But there was no hole or anything in the liner for the fluid to drain out of it. I added the fluid with this liner removed and voila the fluid drained down into the rear brake. Can anyone explain why that liner was in there in the first place? I bought my R1100R last fall, but until now hadn't checked the rear brake reservoir. A link to the photo of the liner is below.

http://www2.snapfish.com/viewsharedphot ... /otsi=SPIC

You will find that the 'liner' compresses down into a small space inside the cap. The brake fluid goes into the reservoir, and then the cap with the collapsed membrane goes on top.
The function of the membrane is to isolate the brake fluid from the air vented into the reservoir through the cap when the level goes down as the pads wear.
The reason it is there is that brake fluid is hygroscopic (absorbs water from the atmosphere), which contaminates the brake fluid. Fresh brake fluid is clear and colorless. Contaminated brake fluid is opaque and dirty brown.
The water in contaminated brake fluid boils at the temperature your brake calipers can reach in heavy braking. When the water boils, you have NO BRAKES.
Brake fluid typically manages to absorb some water, even with the membrane in place, and is contaminated enough to be dangerous in at most two years. You should replace your brake fluid every year to avoid this. Only ever use fresh brake fluid from a previously unopend bottle or can.

HTH.

OK thanks for clearing up that mystery. So when bleeding the brake, I have to remove that liner so the fluid can drain through the system, then when complete, add the brake fluid to the reservoir to the max level, attached the liner (collapsed) on the cap, and screw the cap onto the reservoir?

Terry

Yes. But do be sure to keep the resrvoir topped up while bleeding the brakes (i.e. don't let it get empty.).