How to Bleed a Hydraulic Clutch
There are three methods used by auto manufacturers to actuate the clutch diaphragm in manual transmission vehicles: push rod, cable, and hydraulic. Adjusting push rod and cable-type linkages is fairly simple, but hydraulic clutches require bleeding when installing a new hydraulic throwout bearing. Here’s what you need to know about how to bleed a hydraulic clutch.
Clutch Actuators Types
There are two types of hydraulic clutch actuators: the ram type and the bearing-type.
Ram-type clutch actuators
This system is a hybrid of a push-rod mechanism, where it uses a typical mechanical clutch fork and throwout bearing to operate the clutch diaphragm. The push rod linkage is replaced with a hydraulic system, where the pedal operates a master cylinder which drives a slave cylinder mounted outside the bellhousing. The slave cylinder or ram, moves the clutch fork, operating the clutch engagement.
Bearing-type clutch actuators
Most modern vehicles have gone to the fully-integrated hydraulic system, which eliminates the mechanical fork and bearing and replaces it with a hydraulic throwout bearing. When the clutch master cylinder pressurizes the fluid in the lines, the bearing ram expands, pushing on the clutch diaphragm, disengaging the clutch.
Bleeding the clutch
Bleeding the clutch is a little different from bleeding brakes. The main difference is that there is only point where the fluid is utilized, so air is less likely to be trapped in a T-fitting or caliper. Because of this, the process is very simple and can be handled by one person.
Start off with plenty of fluid in the reservoir. You don’t want the master to run dry, which would cause more headaches.
Locate the bleed port on the ram or the line coming off the hydraulic bearing. If you are working with a bearing, you will need two wrenches, one to hold the line and one to open the bleeder.
Crack the bleeder open and let it drip. Because the fluid is above the master cylinder and the lowest point in the system is the bleeder, it should push all of the air out of the lines and out of the ram or bearing. If this is a new system, you will get a few drops slowly and the speed will increase as the air is expelled until the fluid is consistently dripping. Close the bleeder screw, and top off the reservoir.
Now you can check the clutch pedal. It should be firm, not spongy and it should not change with repeated pumping. If it gets more firm or feels spongy, you need to continue the process.
Sometimes it may be necessary to pressure bleed the system, which is just like bleeding brakes. This involves a second person pressurizing the clutch system and holding the pedal, then cracking the bleeder open. This can be required if the ram or bearing have been disassembled.
With a little patience, you should be able to get your clutch operating at full power with a properly bled hydraulic system. Be sure to use the correct fluid in your clutch system as per the vehicle manufacturer’s specifications.
To learn more about NAPA AutoCare, visit www.NAPAAutoCare.com.