TECH
nichtubermax
tem on his 1996 R 1100 RT. The system will pressurize with no input of the brake lever to the point that the bike is immobilized and brake fluid seeps from the master cylinder. I'm sure if you would let the bike run after it clamps the front wheel it would blow the cover off the master cylinder. We have at this point replaced all lines with braided stainless, rebuilt the master cylinder, blown out ABS module, and fully rebuilt the Brembo calipers, but it’s still not working properly and the bike is indefinitely side- lined. –Doug W. via Facebook
A:
Let’s begin with the assumption that you are not
over the maximum mark for filling the brake fluid reser- voir on the handlebar. It’s important to stay at or below the max mark (remember – NICHT UBER MAX!) because that allows expan- sion room for the fluid as it heats up under normal use. On some older Ducatis, the rear
Digging deeper into brake issues Q:
By George Mangicaro #136221
My father and I have been working on a problem with the front brake sys-
the bike,” as in this happens while you’re riding or is it happening at idle? Since fluid is coming out the master cyl-
inder, this gives me three ideas: the master cylinder itself, the piston that connects the brake lever to the master cylinder and your hard brake lines. The first thing to do is check what you’ve
already done, and in this instance my initial suspicion is the master cylinder rebuild, especially given that it’s an 1100. In early 1100s such as yours, the master cylinder
could have been slightly misadjusted or misaligned. This could partially block the return port for the brake fluid. Effectively this would keep the brake pressurized and the caliper clamped tight to the disc. This would result in the brake fluid heating up and you might see fluid seeping from the master cylinder cover as the pressure and heat cause the fluid to expand and move forcibly. Third (but least likely) is that you have a
There’s a reason BMW puts red paint on the piston moved by the brake lever – they don’t want you to adjust it! If it’s not in precisely the right position, it can cause other internal components to block the fluid return hole and prevent brake fluid from flowing away from the calipers.
brake line was routed so close to the exhaust pipe that it would heat up the fluid, causing it to expand enough to clamp the rear brake tight. This was made worse if the pedal wasn’t prop- erly adjusted, but it was a problem either way. Since you don’t have a Ducati, though, this probably isn’t what’s going on. Thinking about those Ducatis, though, you said “run the bike.” With the Ducatis, this was an issue at idle after the bike was warmed up. Do you really mean “ride
40 BMW OWNERS NEWS January 2017
bore was not anodized, which can promote corrosion and scoring and lead to block- ages. It’s possible when the master cylinder was rebuilt that the bore wasn’t checked or properly cleaned. If there’s debris in there blocking the bore, it would prevent fluid from returning to the master cylinder, thus keeping the calipers clamped down. The second thing it could be is the brake
lever piston adjustment. If the brake lever was replaced or damaged and subsequently not adjusted to spec, then this could be keeping the system pressurized, much as if you were constantly riding the brake. When you rebuilt the master cylinder, the piston
problem with your hard steel brake lines. If they are filling up with plaque— not unlike an old man’s arteries do-then this would create the kind of pressure you’re seeing (as well as excess heat while riding) and cause the brakes to always drag and refuse to release. Upgrading your rubber brake lines to the stainless steel braided ones, while a good upgrade, would not solve this problem. The pressure generated by applying the brakes is subject to the princi- ples of hydraulics in that if you squeeze fluid from a large hose into a smaller hose and that smaller hose is partially blocked with goo, the fluid will move one direction more easily than the
other. So what you could be seeing here is fluid that flows to the calipers goes through the hard lines just fine, but because of pos- sible blockages, that fluid cannot flow back through the hard lines because there is very little pressure moving the fluid in that direction. If you want to look up the prin- ciples of hydraulic multiplication of force, that may explain this concept in more depth.
Have questions for Nicht Uber Max? Send them to
TechQuestions@bmwmoa.org.
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