Check a combined braking system (CBS)

A combined braking system (CBS) links the two brakes: one control also applies part of the other brake. On the K-Light 125, pressing the rear pedal works the rear caliper and, through a separate CBS unit (a second small master cylinder with its own reservoir), the middle piston of the three-piston front caliper. Many scooters do the same with cables and an equaliser at the left lever on drum brakes. The layout differs between makes, so detailed CBS work and bleeding are workshop jobs using the maker's routine. This page lists the safe checks and what to ask for.

Safety

• Do not ride if the pedal or lever reaches its stop, a brake does not release, or fluid leaks anywhere in the system.
• Do not use a simple one-circuit bleed on a CBS bike. The K-Light 125 bleeds the front, rear and CBS circuits separately, and the CBS circuit bleeds at the front caliper. Air left in one circuit makes the linked brake weak or uneven.
• The K-Light 125 manual describes a fail-safe setting of the CBS adjusting bolt for when the linked front circuit fails through a leaking caliper. That makes the rear brake work alone; it is a workshop measure to move the bike, not a repair. Do not ride normally in that state.

What you can check safely

1. Check all reservoirs, including the CBS reservoir, with Check brake fluid level and condition.
2. Check hoses and pipes to the CBS unit and front caliper with Check brake hoses and banjo bolts.
3. With the bike on its stand, press the rear pedal gently while a helper turns the front wheel. On a working K-Light type CBS, the front wheel slows a little. Release: both wheels must spin freely.
4. Check the rear pushrod setting (K-Light 125: 0 to 0.2 mm gap, piston free) with Adjust the rear brake pedal. A pushrod set too long holds the CBS on and makes both brakes drag.
5. Cable CBS (scooters): check the equaliser and both cables move freely and are adjusted to the manual. A stuck cable makes one brake drag or do nothing.

What the workshop will do

• Internal leak test (K-Light 125 method): loosen the lock screw on the CBS adjusting bolt, hold the rear pedal down and try to turn the adjusting bolt by hand. If it turns, there is an internal leak and the CBS unit or its plunger assembly is replaced. If it will not turn, there is no internal leak.
• Strip and inspect the CBS unit, plunger, seals and adjusting valve if needed.
• Bleed every circuit in the maker's order with fresh fluid of the correct type (K-Light 125: DOT4).
• Re-set the pushrod and pedal free play and test both brakes separately and together.

What to ask for

• A CBS function test and internal leak test to the maker's routine.
• A full bleed of all circuits, not just front and rear.
• Replacement CBS unit or plunger kit if the leak test fails.

Check your work

• Pedal firm; front wheel slows a little when only the pedal is pressed; both wheels release fully.
• No leaks; all reservoirs correct.

If it did not fix the problem

• Go to Combined braking (CBS) does not work as it should.

Reference values (K-Light 125)