This reference covers hydraulic brake bleeding and DOT-fluid compatibility for ONYX RCR 72V, CTY2, and 80V systems as identified below. Confirm the fluid specified on the installed brake hardware, particularly on an aftermarket system. Do not apply hydraulic instructions to the stock cable-operated rear brake.
For symptom diagnosis, see ONYX Brake Diagnostics. Retain the caliper-removal and spacer instructions in the relevant front or rear upgrade procedure.
ONYX Brake System Differences (72V vs 80V)
The ONYX 72V, CTY2, and 80V platforms use different braking system designs, which directly affects diagnostics and expected performance.
| Feature | 72V ONYX / CTY2 | 80V ONYX |
|---|---|---|
| Front Brake | Hydraulic (single piston) | Hydraulic (larger / dual piston) |
| Rear Brake | Mechanical cable | Hydraulic |
| Rear Brake Power | Low (by design) | Moderate |
| Brake Feel | Mixed (hydraulic + cable) | Fully hydraulic |
| Bleeding Required | Front only | Front and rear |
Practical Impact
- Weak rear braking on a 72V / CTY2 is normal behavior
- Weak rear braking on an 80V indicates a problem
- 72V relies heavily on regenerative braking for rear deceleration
- 80V behaves closer to a fully hydraulic motorcycle system
Brake Fluid
The ONYX RCR uses DOT 3 or DOT 4 brake fluid only.
Using the correct fluid type is critical for seal compatibility, moisture handling, and consistent braking performance.
Fluid Types
| Fluid Type | Base | Typical Dry Boiling Point | Notes |
|---|---|---|---|
| DOT 3 | Glycol | ~205°C / 401°F | Stock fluid |
| DOT 4 | Glycol | ~230°C / 446°F | Higher boiling point |
| DOT 5.1 | Glycol | ~260°C / 500°F | Compatible but not used in ONYX systems |
| DOT 5 | Silicone | ~260°C / 500°F | Not compatible |
Compatibility Matrix
| From \ To | DOT 3 | DOT 4 | DOT 5.1 | DOT 5 |
|---|---|---|---|---|
| DOT 3 | ✓ | ✓ | ✓ | ✗ |
| DOT 4 | ✓ | ✓ | ✓ | ✗ |
| DOT 5.1 | ✓ | ✓ | ✓ | ✗ |
| DOT 5 | ✗ | ✗ | ✗ | ✓ |
Key Rules
- The bike normally ships with DOT 3
- DOT 3 and DOT 4 can be safely mixed
- DOT 5.1 should not be used in ONYX systems despite compatibility
- DOT 5 must never be used or mixed
Why DOT 5.1 Is Not Used
Although DOT 5.1 is glycol-based and technically compatible:
- provides no meaningful performance gain in this system
- more sensitive to contamination and moisture in real-world use
- unnecessary for the thermal limits of ONYX braking systems
Why DOT 5 Is Not Compatible
DOT 5 is silicone-based, unlike DOT 3, DOT 4, and DOT 5.1 which are glycol-based.
This creates several problems:
- does not absorb moisture (water pools inside the system)
- increases corrosion risk
- incompatible with seals designed for glycol fluids
- cannot mix with DOT 3, DOT 4, or DOT 5.1
Using DOT 5 in an ONYX system can lead to brake failure and long-term damage.
Practical Recommendation
For ONYX use:
- DOT 3 = acceptable and stock
- DOT 4 = preferred upgrade for higher heat tolerance
- DOT 5.1 = do not use in ONYX systems
- DOT 5 = do not use under any condition
Critical Warnings
- Do not use DOT 5
- Do not use DOT 5.1 in ONYX systems
- Do not mix silicone and glycol fluids
- Do not allow oil or grease near pads or rotor
- Do not operate with low fluid levels
Brake Bleeding (Hydraulic System)
When to Bleed Brakes
- soft or spongy lever
- inconsistent braking feel
- after replacing brake fluid
- after opening the brake system
This system is highly sensitive to air; even small amounts will significantly reduce braking performance.
Platform Notes
- 72V ONYX / CTY2 -> bleed front brake only
- 80V ONYX -> bleed front and rear brakes
Tools Required
- DOT 3 or DOT 4 brake fluid
- Clear hose
- Catch container
- 8mm wrench
- Clean cloth
Brake Bleeding Procedure
- Keep the bike upright and stable
- Turn handlebars so the reservoir is level
- Remove reservoir cap
- Attach hose to bleed valve
- Squeeze and hold brake lever
- Open bleed valve briefly
- Close valve before releasing lever
- Release lever
- Repeat until no air bubbles appear
- Maintain fluid level - do not let it run dry
Critical Bleeding Rules
- Never release the lever while the bleed valve is open
- Never allow the reservoir to run dry
- Use only DOT 3 or DOT 4 fluid
- Clean spilled fluid immediately
Completion Check
- Lever feels firm
- No visible air bubbles
- Consistent braking pressure
- Reservoir filled to proper level
If proper lever feel cannot be restored, inspect for leaks or internal seal failure.
Alternative Bleeding Methods
Vacuum Bleeding
- Uses a vacuum pump at the caliper
- Pulls fluid through the system
Notes:
- May pull air past bleed valve threads
- Ensure tight hose seal
- Not always reliable for final bleed
Reverse Bleeding
- Fluid is pushed from caliper to reservoir
- Forces air upward
Notes:
- Tap brake line and caliper lightly to release trapped air
- Monitor reservoir level closely
- Stop if fluid rises too quickly
Recommendation
- Use standard bleeding method first
- Use alternative methods only if needed
- Always finish with a manual lever bleed