Master Cylinder Rebuild Kit for Honda CB350G / CB360 / CB400F / CB450 / CB500T / CB500K / CB550
Complete rebuild kit for the hydraulic front brake master cylinder on the Honda twin and four-cylinder lineup. Kit includes piston & seal cups, return spring hardware, reservoir cap diaphragm, and the brake lever stop bushing — every rubber and metal wear part inside the master cylinder. Gets a seized, leaky, or spongy master cylinder back to factory feel.
Why You Need This
After years of missed fluid changes and time in the barn, the master cylinder on your Honda is stuck. The piston won't return, the rubber cups are glazed or torn, and the diaphragm under the reservoir cap has hardened. The only thing your master cylinder is stopping is progress on your bike.
Honda's own service manual puts it directly: "rubber parts play a critical role. Whenever any damage or deterioration to the rubber parts is noted, they must be replaced immediately" (CB450 FSM p.193). This kit has the parts to do that — piston, seals, spring, diaphragm, and lever stop bushing. If the master cylinder bore itself is too pitted to hone clean, our full master cylinder replacement is the move instead.
Specifications
| Spec | Detail |
|---|
| Replaces OEM Part Numbers | 45530-377-305, 45504-300-003, 45505-300-003, 45506-300-003, 45512-300-003, 45502-300-305, 46125-533-003, 46140-533-003, 90651-377-003, 45520-300-000, 45526-341-000, 45521-MJ6-006, 45521-300-010 |
| Fits Models | CB350G (1973), CB360G/T (1974–1976), CB400F (1975–1977), CB450 K2–K7 (1969–1974), CB500K (1971–1973), CB500T (1975–1976), CB550K (1974–1978), CB550F (1975–1977) |
| Master Cylinder Piston Diameter | 14 mm (matches the factory unit) |
| Brake Fluid Spec | DOT 3 (per Honda CB360 Shop Manual p.29; CB500T FSM p.15; CB550F FSM p.184) |
| Lever Free Play Spec | 2–5 mm (0.08–0.2 in.) at the tip (CB450 FSM p.194; CB550F FSM p.72) |
| Snap Ring Size | 18 mm internal (CB360 Shop Manual p.80) |
| Sold As | Complete rebuild kit |
What's in the Kit
- Master cylinder piston with primary and secondary seal cups
- Return spring + spring retainer
- Reservoir cap diaphragm (rubber)
- Brake lever stop bushing (rubber stopper at the bottom of the piston bore)
- Snap ring
- White plastic cap vent washer — available as an optional add-on at checkout
Note: Does NOT fit the CB750 — CB750 uses a different, larger master cylinder. For the CB750, see our CB750-specific parts.
Installation
Difficulty: Intermediate. If the piston is frozen, plan for penetrating oil and heat cycles before the piston will come out. Budget a couple of hours if it's your first rebuild — the bore has to be clean and smooth, and the reassembly order matters.
Tools needed: 10 mm box wrench, 8 mm box wrench (for bleeder), snap ring pliers or a sharp pick, small-bore cylinder hone, penetrating oil (PB Blaster), clean DOT 3 brake fluid, nitrile gloves, rag to protect paint.
- Keep notes as you disassemble. Photograph each stage and note the orientation of the piston cups, spring, and snap ring — reassembly order is the make-or-break step.
- Remove the master cylinder from the handlebar. Disconnect the banjo bolt at the brake hose — cover surrounding paint; brake fluid damages finishes on contact.
- Remove the brake lever, the oval lever-stop-bushing cover plate (often missing on project bikes), and the rubber boot.
- Free the piston. If the piston is seized, apply penetrating oil and heat-cycle the master cylinder two or three times. Punch the piston out from the banjo bolt side using a wooden dowel.
- Remove the 18 mm internal snap ring. Use a sharp pick if snap ring pliers don't fit — the new kit includes a replacement snap ring, so destroying the old one is fine.
- Hone the master cylinder bore. A small-bore cylinder hone is needed — standard auto-parts-store hones are often too large for this bore.
- Bead-blast or clean the outside of the housing, then wash out the bore with clean brake fluid.
- Install the new spring metal cup first (into the spring), then the spring, then the front piston with the cup facing DOWN toward the spring. The star-shaped metal reed valve on the end of the primary cup is delicate — don't bend it.
- Press the snap ring down into its locking groove. Use a deep-wall socket over the piston so you're pressing on the snap ring, not on the piston. Face the snap ring opening toward the handlebar clamp.
- Install the new rubber boot, lever stop bushing, and lever stop cover.
- Install the new diaphragm into the reservoir cap with the white plastic vent washer BETWEEN the diaphragm and the cap.
- Lubricate the brake lever pivot with light grease. Reinstall the lever.
- Reinstall the master cylinder to the handlebar. Connect the brake hose with fresh copper crush washers on both sides of the banjo fitting.
- Fill with fresh DOT 3 brake fluid and bleed the system. The master cylinder doesn't move much fluid per stroke — bleed at the junction block first, then at the caliper, otherwise you'll squeeze the lever all day getting nothing.
- Check lever free play at the tip: 2–5 mm (0.08–0.2 in.). Spongy lever = air still in the system — re-bleed.
Our two-part video series covers this rebuild end-to-end — Part 1: Disassembly walks through getting a seized master cylinder apart without breaking anything, and Part 2 (above) covers reassembly in the correct order.
Frequently Asked Questions
Does this fit my CB750?
No. The CB750 uses a different master cylinder with a larger piston diameter — this kit will not fit. Separate CB750-specific parts are required.
Does this fit my CB400F or CB350F?
Yes. The CB400F Super Sport and CB350F four-cylinder use the same Tokico master cylinder as the rest of the CB family listed above. This is confirmed in our Master Cylinder Rebuild KB article.
Does this fit my CB450 K2?
Yes. The CB450 K2 (1969) has the hydraulic front disc brake with the same master cylinder used through the K7. The K0 and K1 are drum-brake bikes and this kit is not relevant to them.
How do I know if my master cylinder needs to be rebuilt vs replaced?
Rebuild if: the bore has light surface corrosion that cleans up with a hone, the piston is stuck but the bore isn't scored, fluid is leaking past the seals, or the lever is spongy with no air in the line. Replace the whole master cylinder if: the bore is deeply pitted or scored, the housing is cracked, or the piston bore is out-of-round. Our replacement master cylinder matches the factory 14 mm piston unit.
What brake fluid should I use?
DOT 3. That's the Honda factory spec across the CB360, CB450, CB500T, and CB550 service manuals. Don't mix DOT 3 with DOT 5 (silicone) — the two are not compatible and mixing them will damage the seals.
What's the lever stop bushing for?
It's the rubber stopper at the bottom of the piston bore that limits how far the piston can return. If it's missing or chewed up, the piston can over-extend and the primary cup can pop out of the bore. The oval cover plate above it is easy to lose — check yours before you start.
Do I need the cap vent washer?
The white plastic vent washer sits between the diaphragm and the cap and keeps the diaphragm from getting pulled into the reservoir when the fluid level drops. Honda fit one from the factory. It's available as an optional add-on at checkout — recommended.
You Might Also Need
Last updated: April 2026
