Bleeding a hydraulic clutch bearing, also known as a hydraulic throwout bearing, is the process of removing trapped air from the hydraulic system to ensure proper clutch engagement and disengagement. Air in the system can lead to a spongy pedal feel or incomplete clutch operation.
Why is Bleeding Essential for Hydraulic Clutch Bearings?
A hydraulic clutch system relies on incompressible fluid to transmit force from the pedal to the clutch mechanism. If air, which is compressible, is present in the lines, the force applied to the pedal will compress the air instead of actuating the clutch bearing, leading to poor or no clutch function.
Essential Preparations for Bleeding
Before you begin the on-vehicle bleeding process, consider some crucial preparatory steps, especially bench bleeding.
Bench Bleeding Your Hydraulic Clutch Bearing
As highlighted by Mantic Clutch USA, bench bleeding the hydraulic throwout bearing before installation can significantly simplify the overall bleeding process. "This makes life much much easier if you do this a couple times on the bench. Purge all the air out of the system."
- Process:
- With the bearing off the transmission and connected to the master cylinder (if possible to simulate the system), hold the bearing upright.
- Fill the reservoir with fresh, clean hydraulic fluid (usually DOT 3 or DOT 4 brake fluid, check your vehicle's specifications).
- Slowly actuate the master cylinder rod, allowing fluid to fill the bearing and lines. You may need to manually depress the bearing's piston slightly while cycling the master cylinder.
- Observe for air bubbles escaping back into the reservoir or out of the bleeding port (if accessible on the bench).
- Repeat this process several times until no more air bubbles are visible, ensuring you "purge all the air out of the system." This pre-bleeding will drastically reduce the amount of air you need to remove once the system is installed in the vehicle.
Methods for Bleeding a Hydraulic Clutch Bearing (On-Vehicle)
Once the hydraulic clutch bearing is installed, follow one of these common methods to complete the bleeding process. Always refer to your vehicle's service manual for specific procedures and fluid types.
1. Traditional Two-Person Method (Pump and Hold)
This is the most common method and requires two individuals: one to operate the clutch pedal and one to open/close the bleeder valve.
- Equipment Needed:
- Clear hose that fits snugly over the bleeder valve.
- Container to collect old fluid.
- Wrench for the bleeder valve.
- Fresh, clean hydraulic fluid.
- Steps:
- Ensure the hydraulic fluid reservoir is full.
- Attach the clear hose to the bleeder valve on the hydraulic throwout bearing (or slave cylinder, if applicable) and place the other end into the collection container submerged in a small amount of fresh fluid.
- Person 1 (in vehicle): Slowly pump the clutch pedal 3-5 times, then hold it firmly to the floor.
- Person 2 (at bleeder valve): Briefly open the bleeder valve to allow fluid and air to escape, then quickly close it.
- Person 1: Release the clutch pedal.
- Repeat steps 3-5 until no more air bubbles are visible in the clear hose, and the fluid coming out is clean and clear.
- Maintain the fluid level in the reservoir throughout the process.
2. Reverse Bleeding Method
This method involves forcing fluid up from the bleeder valve to the master cylinder reservoir, often effective for stubborn air pockets.
- Equipment Needed:
- Large syringe or fluid pump.
- Clear hose.
- Fresh hydraulic fluid.
- Steps:
- Ensure the reservoir cap is removed.
- Attach a hose from the syringe/pump to the bleeder valve.
- Fill the syringe with fresh hydraulic fluid.
- Slowly and steadily push fluid from the syringe into the bleeder valve. Air bubbles will rise into the reservoir.
- As the reservoir fills, some fluid and air will overflow. Have a rag or container ready to catch it.
- Continue until no more bubbles appear in the reservoir, and the clutch pedal feels firm.
- Close the bleeder valve, re-install the reservoir cap, and test the pedal.
3. Vacuum Bleeding Method
A vacuum pump can pull fluid and air out of the system from the bleeder valve, often making it a one-person job.
- Equipment Needed:
- Hand-held vacuum pump with reservoir.
- Wrench for bleeder valve.
- Fresh hydraulic fluid.
- Steps:
- Fill the master cylinder reservoir.
- Attach the vacuum pump's hose to the bleeder valve.
- Open the bleeder valve.
- Operate the vacuum pump to draw fluid and air into its reservoir.
- Monitor the fluid level in the master cylinder reservoir and refill as needed to prevent air from being drawn in.
- Continue until no more air bubbles appear in the vacuum pump's collection reservoir.
- Close the bleeder valve, remove the vacuum pump, and test the pedal.
Troubleshooting Common Bleeding Issues
| Issue | Possible Cause | Solution |
|---|---|---|
| Spongy Pedal | Air still trapped in the system | Re-bleed using any of the methods; try reverse or vacuum bleeding. |
| No Pedal Pressure | Major air pocket, master cylinder issue, leak | Check for leaks. Ensure reservoir didn't run dry during bleeding. |
| Fluid Not Flowing | Clogged line, stuck bleeder valve, closed valve | Check for obstructions. Ensure bleeder valve is open when bleeding. |
| Fluid Discoloration | Old, contaminated fluid | Continue bleeding until clean fluid appears. |
| Repeated Bleeding Needed | Leak in the system | Inspect all connections, lines, and cylinders for leaks. Replace faulty components. |
By following these methods and incorporating the crucial step of bench bleeding, you can effectively purge air from your hydraulic clutch bearing system, restoring a firm and responsive clutch pedal.
