A combined braking system (CBS) links the brakes, so one control also applies part of the other brake. On the K-Light 125 the rear pedal also works the middle piston of the front caliper through a small CBS unit. On many scooters the left lever pulls both drum brakes through an equaliser. A fault shows as: the linked brake does nothing, one brake drags after use, the pedal sinks or travels far, or the bike pitches or skids when you use one control.
Is it safe to ride?
No. A CBS fault changes how the bike stops and can make a wheel drag or lock.
Quick checks first
- Check all reservoirs, including the CBS reservoir, are above LOWER.
- Look for fluid on the CBS unit, its pipes and the front caliper.
- Spin both wheels on the stand after pressing the rear pedal (or CBS lever) and releasing it.
- Scooter cable CBS: are both cables attached and free?
Diagnose it step by step
1Is the system hydraulic (CBS unit with reservoir and pipes) or cable?
- Cable: go to step 6.
- Hydraulic: go to step 2.
2Is any reservoir low, or is there a leak?
- Yes: go to Brake fluid is leaking and Check brake hoses and banjo bolts.
- No: go to step 3.
3After you press and release the pedal, does either wheel stay braked?
- Yes: the rear pushrod may be too long and holding the pistons. Adjust the rear brake pedal (K-Light 125: 0 to 0.2 mm gap, piston free). If it still drags, Check for brake drag.
- No: go to step 4.
4With the bike on its stand, does the front wheel slow a little when only the pedal is pressed?
- No: the CBS circuit has air, a blocked line or an internal fault. Workshop: Check a combined braking system (CBS).
- Yes: go to step 5.
5Does the pedal sink slowly when held?
- Yes: the workshop does the K-Light internal leak test (adjusting bolt turns by hand while the pedal is held means an internal leak; the CBS unit or plunger is replaced). See Check a combined braking system (CBS).
- No: have the workshop bleed all circuits in the maker's order. A simple one-circuit bleed misses the CBS circuit.
6Cable CBS: do both cables and the equaliser move freely and return?
- No: Lubricate control cables or replace a frayed cable, then adjust both brakes to the manual.
- Yes: adjust each brake to its own free play first, then the linked lever. Check the drum shoes if a brake is still weak: Drum brake is weak.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Pushrod or pedal set wrong | Drag after pedal use; free play off | Adjust the rear brake pedal |
| Low fluid or leak | Low reservoir; wet pipes | Check brake hoses and banjo bolts |
| Sticky cable or equaliser (cable CBS) | Stiff lever; one brake drags | Lubricate control cables |
| Air in CBS circuit | Front does not respond to pedal | Check a combined braking system (CBS) |
| CBS unit internal leak | Pedal sinks; leak test fails | Check a combined braking system (CBS) |
Reference values (K-Light 125)
| Item | Value | Source |
|---|---|---|
| Rear pedal free play | 20 to 30 mm | K-Light 125 manual, PDF p. 62 |
| Pushrod to rear master piston clearance | 0 to 0.2 mm | K-Light 125 manual, PDF p. 116 |