Short version first: this is a full walk‑through to identify, remove and replace the clutch slave cylinder on a Hino Super F Series GH1H (covers both common external slave and concentric slave types), why it’s done, how the clutch hydraulic system works, what to inspect, how to bleed it correctly, and common failure modes. Follow the safety steps. Use the truck’s OEM workshop manual for exact torque values and the specified hydraulic fluid.

Theory — how the system works (simple analogy)
- Think of the clutch hydraulic system like the heart and a remote muscle. The clutch master cylinder (MC) is the heart: when you press the pedal, the MC pushes hydraulic fluid (incompressible like water) down a line. The slave cylinder (SC) is the muscle at the clutch: fluid pressure pushes its piston, converting fluid pressure into a mechanical push that moves the clutch release mechanism (fork or concentric bearing) to disengage the engine from the transmission.
- Because hydraulic fluid cannot be compressed like air, the motion is positive and proportionate — if air gets in the line you get a spongy pedal because air compresses.
- Why replace the slave cylinder: typical failures are internal seal wear (piston leaks internally -> no pressure), external leaks (fluid leaks onto the ground or into bellhousing), seized piston (won’t move), or ruined rubber boot. Symptoms: soft/spongy pedal, clutch not releasing fully, pedal slowly sinking to the floor, visible fluid leak, contamination of bellhousing.

Main components you will encounter (detailed)
- Clutch pedal and pushrod: the pedal arm inside the cab connects to the clutch MC pushrod.
- Master cylinder (MC): reservoir on top, piston assembly below. Sends pressure to line.
- Hydraulic line/hoses: metal line and rubber hose that carries fluid from MC to SC. Fittings: banjo bolts or flare fittings; copper crush washers on banjo.
- Reservoir cap & fluid: contains DOT-specified hydraulic fluid.
- Slave cylinder (SC) — two types:
- External/slave cylinder (typical older trucks): mounted on bellhousing, has piston and pushrod that contacts clutch fork or release lever.
- Concentric slave cylinder (CSC) / internal: sleeve-shaped unit inside transmission around input shaft; when it fails you usually must remove transmission to replace it.
- Bleed nipple/bleeder valve: small valve on SC for bleeding air.
- Clutch fork and release (throw-out) bearing: mechanical parts that transfer SC motion to the clutch pressure plate.
- Seals, O-rings, dust boot: keep fluid in and dirt out.
- Mounting bolts, dowel pins, and gaskets.

Tools and parts checklist
- New slave cylinder (correct part for GH1H; confirm external vs concentric)
- New copper crush washers (banjo) or new flare fitting parts if required
- Correct hydraulic fluid (manufacturer spec — commonly DOT 3 or DOT 4; check manual)
- Basic hand tools: socket set, ratchet, wrenches (including line wrenches), screwdrivers, pliers
- Torque wrench (for critical bolts)
- Jack and jack stands or ramp (truck-level lifting equipment), wheel chocks
- Drain pan & rags, shop towels
- Bleeder hose and clear plastic tube or vacuum bleeder or pressure bleeder
- Protective gloves, eye protection
- Brake/clutch parts cleaner
- Penetrant (if fasteners are seized)
- Plastic plug or cap to prevent dirt entering lines once disconnected
- Replacement hydraulic line if corroded
- Service manual for torque specs and removal illustrations

Safety first
- Park on level ground, chock wheels, engage parking brake, place vehicle in gear or park.
- Use rated jacks and jack stands — support the truck securely. Do not rely on hydraulic jack only.
- Wear eye protection and gloves. Brake/clutch fluid damages paint and is harmful.
- Contain fluid: catch and dispose of used fluid properly (hazardous waste).
- Disconnect battery if working near electrical parts or to prevent accidental starting while under vehicle.
- If the slave is concentric (inside transmission), plan for transmission removal — it’s a heavy job; consider a shop.

Diagnosis — confirm slave cylinder is the problem
- Visual leak at the slave or fluid under bellhousing? Strong sign.
- Pedal sinks slowly to floor with steady pressure? Master cylinder may be bad (internal leak), but also indicates system leak.
- Pedal spongy and pumpy even after bleeding? Check for air, or failing MC/SC.
- Does the slave piston move when you press pedal (have helper press while you watch)? If no movement and hydraulic pressure present, SC may be seized or internal leak.

Procedure A — External slave cylinder replacement (most straightforward)
1) Prep:
- Park, chock wheels, jack and support the truck so you can access the bellhousing area where the SC is mounted. Disconnect battery if desired.
- Remove any engine covers, intake or wiring obstructing access.
- Clean area around the slave and bleeder to avoid contamination falling into lines.

2) Protect and drain:
- Place a drain pan under the slave. Unscrew the reservoir cap to relieve pressure.
- If possible, use a turkey baster or syringe to remove excess fluid from the reservoir (minimizes spillage).
- Have rags ready.

3) Disconnect hydraulic line:
- Identify banjo fitting or line connection to the slave.
- Use a line wrench to hold the fitting and loosen the banjo/line nut to avoid rounding.
- Catch fluid with pan; plug open line quickly with a clean cap or rag to reduce contamination.
- Remove old crush washers from banjo bolt (replace them).

4) Remove slave mounting bolts:
- Remove bolts that hold SC to bellhousing. Note any dowel pins or bracket orientation for correct reinstallation.
- Withdraw SC gently; if it’s stuck, apply penetrating oil and tap lightly; avoid forcing and avoid dropping anything into bellhousing.
- Inspect the pushrod area and dust boot. If fluid leaked into bellhousing, inspect clutch disc for contamination (if disc is soaked, clutch replacement may be required).

5) Compare old vs new:
- Compare new SC to old one: mounting holes, pushrod length, bleeder orientation. Make sure bleed nipple points up or in the recommended direction to evacuate air.

6) Install new slave:
- Fit new crush washers on banjo bolt (if applicable).
- Mount SC in position, insert bolts, hand-tighten, then torque to OEM spec (consult manual).
- Reattach hydraulic line; tighten banjo or flare fitting to spec. Do not overtighten and do not cross-thread.

7) Refill reservoir:
- Fill reservoir with correct fluid to the specified level.

8) Bleed the system (detailed below).

9) Final checks:
- Check for leaks at fitting and bleeder.
- Test pedal feel — should be firm when bled correctly.
- With engine off, have helper press pedal several times and hold while you watch bleeder for bubbles. Then start engine and test clutch engagement with care.
- Lower vehicle after confirming no leaks and proper operation.

Procedure B — Concentric slave cylinder (CSC) replacement (if your GH1H uses CSC)
- If the slave is concentric, it sits inside the transmission around the input shaft. Replacement generally requires removal of the transmission:
- Support engine and transmission as per service manual.
- Disconnect driveshaft, linkages, wiring, and transmission mounts.
- Remove transmission from bellhousing. This is a heavy and multi-step procedure — follow the manual, use a transmission jack, and mark alignment dowels.
- Once transmission removed, access CSC on the rear of engine/transmission mating surface. Remove mounting bolts, disconnect hydraulic line.
- Replace CSC (and often recommended to replace the pilot bearing or seal if worn).
- Reinstall transmission, torque bolts to spec, refill/clutch bleed.
- Because of transmission removal complexity, many beginners have this done in a shop unless experienced and equipped.

Bleeding the system — methods and step-by-step
Goal: remove all air from hydraulic line so the pedal is firm.

Method 1 — Two-person pump-and-open bleed (common)
1) Fill reservoir fully before starting and keep it topped up throughout.
2) Attach clear hose to bleeder nipple and put other end into fluid-filled container.
3) Have helper sit in cab and press pedal 8–10 times, then hold pedal to the floor.
4) Open bleeder valve (1/4 turn) — fluid and air will flow out. Close bleeder when flow stops.
5) Helper releases pedal once bleeder is closed.
6) Repeat: pump pedal 6–10 times, hold, open bleeder, close, until fluid runs clear with no bubbles and pedal feels firm.
7) Keep reservoir topped so it never runs dry (introduces air back).
8) Final: tighten bleeder to spec, check pedal for firmness.

Method 2 — Vacuum bleeder (one person, faster)
- Connect vacuum bleeder to bleeder nipple, pump vacuum, open bleeder and let vacuum pull fluid/air out until no bubbles, close bleeder, stop vacuum. Continue until firm pedal. Keep reservoir topped.

Method 3 — Pressure bleeder (shop tool)
- Pressurize reservoir per manufacturer (low pressure) and open bleeder until clear, then close.

Bench bleeds are for master cylinders only — not generally needed for slave replacement, but if you had to open the master cylinder, bench-bleed MC before reinstalling.

Testing after bleed
- Pedal feel should be firm with minimal travel. If pedal is spongy, repeat bleeding and inspect for leaks or a failing master.
- With engine started, try engaging gears in a safe spot: ensure clutch disengages and engages smoothly without slip or grabbing.
- Check for leaks under the vehicle and at all fittings after a short test drive.

What can go wrong — common failure modes and mistakes
- Introducing air by letting reservoir run dry while bleeding -> spongy pedal; repeat bleeding.
- Cross-threading banjo/line fittings or overtightening -> damaged threads or crushed fittings -> leaks.
- Forgetting new crush washers -> leaks at banjo joint.
- Contaminating clutch friction surfaces if fluid leaks inside bellhousing -> clutch slipping; may require clutch replacement.
- Damaging the dust boot or pushrod when installing -> accelerated wear or contamination.
- Using wrong fluid (mixing DOT 5 silicone with DOT 3/4 glycol) -> seal damage; always use specified fluid.
- Not torquing bolts to spec -> loose slave -> leaks or misalignment; over-torque -> stripped threads.
- If the master cylinder is failing (internal leak), replacing slave alone won’t fix symptoms.
- For CSC replacement: incorrectly aligning transmission dowels or not supporting engine properly -> dangerous, difficult reassembly, driveline misalignment.
- Breaking bleeder screw by rounding it off — use correct wrench and don’t apply excessive force.

Inspection and maintenance tips
- Whenever slave is replaced, inspect master cylinder, reservoir cap vent, rubber hoses, and fittings. Replace hoses if cracked.
- Replace crush washers every time you remove a banjo.
- If fluid looks dirty, flush the system.
- Keep the area clean to avoid dirt entry.
- If bellhousing shows fluid contamination, inspect clutch friction disc for oil/contamination — oil-soaked discs need replacement.

Signs that replacement didn’t fix the issue
- Pedal still soft: check for air, or failing master cylinder.
- Clutch slips after repair: check for contamination, worn clutch, improper adjustment (if manual adjustment available).
- Leaks return: bad fitting, cracked line, or wrong part.

Estimated time and difficulty
- External slave replacement: moderate difficulty, 1–3 hours depending on access and corrosion.
- CSC (concentric) replacement: advanced; requires transmission removal — plan for many hours or shop service.

Final checklist before declaring job done
- No leaks at line/fittings/bleeder.
- Pedal firm and has correct free play (per manual).
- Clutch fully engages and disengages through gears.
- All tools removed, reservoir cap installed, fluid level correct.
- Dispose of used fluid safely.

Closing note
- This guide gives the complete approach and cautions for a Hino Super F Series GH1H. For exact torque values, fluid specification, and vehicle-specific removal diagrams, use the Hino workshop manual for GH1H. If your truck uses a concentric slave cylinder and you are inexperienced with transmission removal, consider professional help.

No further questions.
rteeqp73

hino with brake switch problems loosing power truck_diagnostic truck_diagnostic.

1992 Hino GS & 1991 Hino FS Trucks - Old School Test Drive Join me as i take you back in time 30 year's with a test drive in a 1991 Hino FS Tipper Truck and also check out a 1992 Hino GS ...

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