The original ABS II system on early BMW R1100S models is relatively straightforward compared with the later servo-assisted iABS system.
There is no electric brake servo, no separate control circuits and no complicated reservoir arrangement under the tank. But there is one important difference from a non-ABS bike:
You must bleed the ABS II pressure modulator as well as both brake calipers.
Ignore the modulator and old fluid can remain inside the ABS unit. Introduce air into the system and it can stay trapped there.
This guide applies to the early ABS II system only. It does not apply to later servo-assisted Integral ABS / iABS bikes.
Why This Matters
Brake fluid absorbs moisture over time. That lowers its boiling point and encourages corrosion inside the master cylinders, calipers and ABS modulator.
BMW specified DOT 4 brake fluid and an annual brake-fluid change for the R1100S.
On an older motorcycle, regular fluid changes are cheap insurance. The ABS II system is robust, but its modulator is not a part you want to neglect.
What You Need
- Fresh DOT 4 brake fluid
- Brake bleeder or vacuum bleeder
- Clear hose and suitable catch bottle
- 8 mm spanner for caliper bleed nipples
- Clean rags
- Brake cleaner
- Gloves
- A way to keep the bike upright and level
A vacuum bleeder is not essential, but it makes this job faster and much easier when working alone. BMW’s own workshop procedure describes vacuum extraction at the calipers.
Before You Start
Brake fluid damages paint.
Cover the tank, fairing and painted panels with clean rags. Keep the reservoir topped up throughout the job. Never allow it to run dry, or you will introduce air into the system and create more work for yourself.
Use only fresh fluid from a sealed container.
The Correct Bleeding Order
For an ABS II-equipped R1100S, bleed each circuit in this order:
Front brake
- ABS pressure modulator — front circuit, marked VR
- Right front caliper
- Left front caliper
Rear brake
- ABS pressure modulator — rear circuit, marked HR
- Rear caliper
BMW specifies that the ABS pressure modulator is bled before the respective caliper circuit.
Front Brake Circuit
Start with the front brake reservoir. Turn the handlebars until the reservoir sits level, remove the cover and diaphragm, and fill to the MAX mark with fresh DOT 4.
1. Bleed the front ABS II modulator
Locate the front-circuit bleed nipple on the ABS pressure modulator, marked VR.
Attach the vacuum bleeder or clear hose, open the nipple around half a turn and draw fluid through until it runs clear and free of bubbles.
Close the nipple before removing the hose.
BMW specifies 9 Nm for the pressure-modulator bleed screw.
2. Bleed the right front caliper
BMW’s original procedure uses the right-hand caliper’s filler adapter. This is the caliper with the extra fitting and grub screw.
Remove the grub screw, connect the bleed equipment, open the bleed point and flush until the fluid is clean and free of bubbles.
Do not let the reservoir fall below MIN.
When finished, refit and tighten the grub screw. The bike must not be ridden with it missing or loose.
3. Bleed the left front caliper
Move to the left caliper, attach the bleeder and flush until the fluid runs clean and bubble-free.
Close the nipple, remove the hose and clean away any spilled fluid.
BMW specifies 14 Nm for the front-caliper bleed screws.
4. Refill and check
Refit the front reservoir diaphragm and cap. With the bike upright and handlebars centred, the fluid level should be at MAX with new pads, or no lower than MIN with worn pads.
Rear Brake Circuit
Top up the rear reservoir with fresh DOT 4 before starting.
1. Bleed the rear ABS II modulator
Locate the rear-circuit bleed nipple on the ABS modulator, marked HR.
Attach the vacuum bleeder, open the nipple around half a turn and flush until the fluid is clean and there are no bubbles.
Close the nipple before disconnecting the hose.
2. Bleed the rear caliper
For best results, remove the rear caliper, remove the inner brake pad and hold the caliper level while bleeding. Push the pistons fully back before flushing the circuit.
Attach the bleeder, open the nipple and draw fluid through until it runs clear and bubble-free.
Close the nipple, reinstall the caliper and check the rear reservoir level.
BMW specifies 6 Nm for the rear-caliper bleed screw.
Vacuum Bleeding Versus Pump-and-Hold
The traditional method works:
- Pump the brake lever or pedal.
- Hold pressure.
- Open the bleed nipple.
- Close it before releasing the lever or pedal.
- Repeat.
But a vacuum system is usually quicker, cleaner and easier for one person.
Use gentle, steady vacuum. Do not obsess over every tiny bubble in the clear hose: vacuum bleeders can pull air past the bleed-nipple threads and make bubbles that are not actually inside the brake system. What matters is a firm lever or pedal, clean fluid and no continuing air emerging from the circuit.
The Overnight Lever Trick
Once the system is fully bled and the front brake lever feels firm, you can turn the handlebars fully left and hold the front brake lever back with a zip tie overnight.
This can allow any very small remaining air bubbles to rise toward the master cylinder.
It is a useful final check, not a substitute for correct bleeding. If the lever is still soft the next morning, bleed the circuit again rather than assuming the zip tie will solve it.
Final Checks
Before riding:
- Check both reservoirs are correctly filled.
- Confirm every bleed nipple is tight.
- Check the front caliper filler-adapter grub screw is fitted.
- Wipe away all brake fluid.
- Check for leaks around the modulator, calipers and hoses.
- Pull the front lever firmly several times.
- Press the rear pedal firmly several times.
- Confirm the ABS warning lights complete their normal self-test once the bike is moving.
For the original R1100S ABS system, both ABS warning lights should go out once the motorcycle reaches approximately 5 km/h.
Bottom Line
ABS II is not difficult to maintain, but it must be treated as more than a normal caliper-only brake bleed.
Follow the sequence above: front modulator, front calipers; rear modulator, rear caliper. Use fresh DOT 4, keep the reservoirs topped up and do not rush the final checks.