Q:

Dear Nicht Uber Max: I bled the brakes on my RT, and they’re still mushy. Every now and then the ABS lights come on, but turning the bike off and back on again usually clears that up. Did I do something wrong when I bled the brakes? –Mike Z., R 1150 RT

A:

The process for bleeding brakes used to be relatively simple. Remove the master cylinder reservoir lid (carefully), protect any paint from splashed brake fluid, and then squeeze the brake lever (or depress the pedal) while a helper opens the bleeder valve on the caliper briefly. Repeat until clear brake fluid flows through the hose on the bleeder valve without any air bubbles present.

With the advent of ABS on motorcycles, followed by BMW’s infamous “whizzy” (servo-assisted) brakes, this doesn’t quite do the trick any more. This is a hydraulic-over-hydraulic system, and the brake fluid in each part of the circuit exists in two separate pools.

One pool goes from the input cylinder (master cylinder) reservoir on the handlebar (for the front brakes) or frame (for the rear brake) to the ABS control unit under the fuel tank. The other pool goes from the ABS control unit to the calipers. Flushing the system to put new fluid in it or simply bleeding the brake lines thus requires you to address both pools of fluid. (Which bike you have determines the total number of pools of fluid you have; for instance, the K 1200 LT bikes with these types of brakes generally have three pools, but depending on the year of manufacture, they could have four.)

Top view of an ABS control unit from an R 1150 RT. The battery is at the bottom of the photo and the bike’s Motronic computer is at the top. Note the combination of rubber and metal brake lines.

My guess—without having access to the R 1150 RT of course—is that Mike bled the system in the old-fashioned way, by opening the bleed valve on the caliper and squeezing the brake lever. Effectively what he did was empty fluid from the ABS unit-to-caliper pool, without replacing it because the fluid level in the “master cylinder” (on the handlebar for the front brakes or on the frame for the rear brake) reservoir would not have decreased.

To flush or bleed these systems, the following process may prove to be the most effective, but as always you should consult with a qualified BMW tech if you’re at all confused.

You’ll need:

• Catch basin for contaminated (old) brake fluid
• A big, unopened bottle (quart/liter) of fresh DOT 4 (or synthetic equivalent) brake fluid. This leaves you plenty of extra in case you make a mistake, spill some, etc. DO NOT use DOT 5 or silicone-based brake fluid.
• Combination wrenches suitable for your bleed valves on the calipers and the ABS control unit (sizes may vary)

Once you have access to the ABS control unit (you may have to remove body panels, seat, fuel tank, etc.), be sure to protect your frame and any other painted parts from spilled or splashed brake fluid – that stuff will take the paint right off. The system is under pressure, so fluid can spray out at high speed.
Attach a hose or tube to the rear caliper bleed valve and run that hose to your catch basin.

The tall, capped fittings are the bleed valves for the ABS control unit. The short, angled, capped fittings (only one is visible) are for getting air out of the lines, but they are reversed from the fluid bleed valves – that is, the one that looks like it should be for the rear brake circuit is actually for the front brake circuit. It is very easy to forget this, which is why it’s advisable to go to a qualified tech if you get air in the system.

• Turn the key on.
• Remove the cap on the reservoir marked H (Hintere, or Rear) and top off the fluid in the ABS control unit reservoir if necessary. (see photo)
• Open the bleed valve a quarter turn.
• GENTLY apply the rear brake and allow fluid to drain, ensuring that fluid in the reservoir is always visible.
• Release the brake and close the bleed valve, then turn the key off.
• Top off the reservoir if necessary.
• Repeat above steps as necessary until you see clear fluid coming through the hose.

Put the wrench on the caliper bleed valve before affixing the hose to the catch basin. Alternately, you can use the open end, but doing it this way helps ensure you don’t over-tighten the bleed valve.

Using a funnel that screws into the ABS control unit reservoir can speed this process, as it gives you a good visual on when you need to add brake fluid. Beemer Boneyard sells them for about $35, or you can get the official BMW Motorrad tool, which is about $225 (p/n 83 30 0 402 174). The funnel also helps prevent air from getting into the system, which is important to both feel and function. A bent 7 mm combination wrench will also come in handy.

For the front calipers, the procedure is the same as the rear, making sure (of course) that you are filling the appropriate reservoir (V for Vorwärts or Forward) on the ABS control unit. Popular opinion says to start with the caliper at the end of the circuit, but if you’re doing both at the same time (and you should), it doesn’t matter which one you do first. Just be sure to only work on one caliper at a time, completing that one before moving on to the next.

Remember that brake fluid can and will damage the paint on your motorcycle. Always protect painted surfaces when working with brake fluid and clean up spills immediately. When you’re bleeding any part of the circuit, ensure the reservoir never runs dry, or you risk introducing air to the system.

Once you’ve flushed and bled both halves of the system from the ABS control unit, you’re only halfway done. Now you can move on to the fluid that starts at the brake lever or pedal.

Start at the rear again:

• Open the cap on the frame-mounted master cylinder reservoir.
• Connect the hose for your catch basin to the tall bleed valve on the ABS control unit.
• Top off the frame-mounted reservoir with clean, fresh DOT 4 brake fluid.
• Open the bleed valve a quarter turn.
• Press firmly on the brake pedal.
• While holding the pedal down, close the bleed valve.
• Release the pedal and refill fluid in the reservoir as necessary.
• Repeat these steps until clear fluid flows through the hose.

Once you’ve flushed and bled the rear, you can move to the front. Follow the same procedure for however many front calipers you have. If you somehow introduce air into the system, things get more complicated, and at that point, it’s advisable to seek professional help.

For hard-to-reach places, cleanup can be accomplished with spray brake cleaner, but always check the labels to make sure whatever cleaner you use won’t damage rubber, paint or plastic. Using lots of water may be a better cleanup solution. Use a drip tray to control run-off and help protect the environment.

The important thing to remember when dealing with the ABS control unit is that the brake fluid connected to the lever or pedal pushes on a piston inside the ABS control unit. The piston inside the control unit then pushes fluid to the caliper—two separate, distinct systems that work together to bring you quickly and safely to a full stop. The core of Mike’s problem was a simple misunderstanding of how the brake system on his bike manages fluid.

If you’re having a problem with your motorcycle that you’d like to see addressed, send your questions or issues to techquestions@bmwmoa.org, and we’ll address your problem in a future installment.