Description and Operation Information
FIGURE 5:
DESCRIPTION
The clutch MASTER CYLINDER performs the following functions, Figure 5:
^ Moves fluid through the hydraulic line to the slave cylinder.
^ Compensates for temperature change and minimal fluid loss to maintain the correct fluid volume by use of the bleed port and compensation port.
^ Compensates for a worn clutch disc and pressure plate by displacing fluid through the reservoir bleed port. This eliminates the need for routine adjustments.
Reserve hydraulic fluid necessary to allow the master cylinder to perform the above functions is supplied to the master cylinder through the fluid supply reservoir. The reservoir is connected to the master cylinder by the "bleed" and "compensation" ports.
The clutch SLAVE CYLINDER performs the following functions, Figure 5:
^ Disengages the clutch by extending the slave cylinder pushrod. This is done by the hydraulic fluid moving from the master cylinder to the slave cylinder which causes the slave cylinder pushrod to extend.
^ Makes sure the clutch release bearing is contacting the clutch pressure plate. This is done by the slave cylinder preload spring.
Hydraulic fluid is supplied to the slave cylinder from the master cylinder through the flexible hydraulic line.
OPERATION
Depressing the clutch pedal moves the clutch master cylinder pushrod. The master cylinder push rod moves the master cylinder piston. The initial movement of the master cylinder piston takes up the "slack" in the clutch system. After the "slack" is gone, the hydraulic fluid moves from the master cylinder to the slave cylinder which causes the slave cylinder pushrod to extend and disengage the clutch. As long as there are no obstructions or blockages in the hydraulic system, the hydraulic pressure is equal throughout the system. This is because hydraulic fluid is not compressible.
FIGURE 6:
FIGURE 7:
CLUTCH PEDAL INITIAL POSITION
When the clutch pedal is in the full released position (contacting the pedal upstop), the master cylinder piston is held against the back of the master cylinder by the piston return spring and the clutch pedal overcenter spring, Figure 6. While the master cylinder piston is in this position, the compensation port is open between the reservoir and the rest of the system. The open compensation port lets hydraulic fluid pass between the reservoir and the other system components. This allows for changes in fluid volume that may be caused by temperature change, minimal fluid loss and/or clutch disc wear. This is commonly known as the self-adjusting feature. The self-adjusting feature will not correct for underdash mechanisms that are out of adjustment.
CLUTCH PEDAL PARTIALLY APPLIED
The initial movement of the clutch pedal will "take up" system clearances before the master cylinder piston moves to build-up hydraulic pressure. These clearances are made up of clutch "lash", master cylinder piston return spring force and friction within the entire system. Enough movement of the master cylinder piston to allow the cup seal to "close off" the compensation port must occur before hydraulic pressure is generated in the system.
FIGURE 7:
FIGURE 6:
CLUTCH PEDAL FULLY APPLIED
When the clutch pedal is fully applied the master cylinder piston travels enough to "close off" the compensation port which allows a build-up of hydraulic pressure that is necessary to move the slave cylinder piston, Figure 6. The slave cylinder piston in turn moves the slave cylinder pushrod which disengages the clutch.
CLUTCH PEDAL RELEASED
With the clutch pedal released, the clutch master cylinder piston return spring positions the piston to its released position. Friction and inertia in the hydraulic system causes the clutch master cylinder piston to return to its initial position faster than the clutch slave cylinder piston. This creates a (partial) vacuum in the clutch master cylinder which allows the lip of the cup seal to pull away from the master cylinder wall. When this happens, hydraulic fluid is
drawn into the master cylinder from the reservoir through the bleed port. As the clutch slave cylinder piston returns to its released position, the hydraulic fluid returns to the clutch master cylinder from the rest of the system. The excess hydraulic fluid in the master cylinder returns to the reservoir through the compensation port.
FIGURE 7:
CLUTCH RELEASE BEARING LUBRICATION
Lubrication of the clutch release bearing is recommended to minimize the possibility of the bearing binding due to contaminants. The following lubrication procedure should be used at every oil change interval. Whenever the transmission is removed for service, a complete cleaning and inspection of the release bearing should be completed as described in the Light Truck Shop Manual, Section 16-01-5.
1. Remove the clutch hydraulic slave cylinder from the transmission bell housing.
2. Remove the clutch release lever and rubber shield from the bell housing.
3. Using a long screwdriver, apply Multi-Purpose Long-Life Lubricant, C1AZ-19590-BA to the bearing retainer through the bell housing opening.
4. Slide the release bearing on the retainer to spread the grease.
5. If any binding is present after performing Steps 1 through 4, replace the release bearing.
HYDRAULIC CLUTCH SYSTEM CONCERNS
^ AIR IN SYSTEM
Air could enter the system through the compensation and bleed ports if the hydraulic fluid level in the reservoir is too low, or if a seal happens to leak. The system MUST be bled to remove the entrapped air.
^ BLEED PROCEDURE
Fill master cylinder with brake fluid.
Disconnect slave pushrod from fork.
Allow slave to fully extend and then fully compress by hand several times.
Disconnect slave from mounting and tip upward while slowly compressing to remove last bit of air.
^ LEAK AT CLUTCH MASTER CYLINDER PISTON SECONDARY SEAL OR SLAVE CYLINDER PISTON SEAL
Hydraulic fluid will escape from the system and allow the entry of air. External leaks will require the replacement of the clutch master cylinder and/or clutch slave cylinder.
^ LEAK AT CLUTCH MASTER CYLINDER PISTON PRIMARY SEAL (INTERNAL LEAKAGE)
This is caused by a worn seal or pitted cylinder wall. External evidence is not visible. A "soft" clutch pedal or in an extreme case, low clutch system pressure that will not release the clutch indicates an internal leak.
^ BLOCKED COMPENSATING PORT
Hydraulic fluid contamination, swollen cup seal, master cylinder piston that does not retract all the way or a pushrod that is the incorrect length. A swollen cup seal usually indicates mineral oil contamination. The cup seal is compatible with brake fluid only. The clutch will eventually slip if not corrected. The clutch master cylinder will need to be replaced if adjustment of the underdash mechanism does not correct the concern.
NOTE: REFER TO THE FOLLOWING TROUBLE SHOOTING GUIDE AND FLOW CHARTS FOR ASSISTANCE IN DIAGNOSING CLUTCH SYSTEM CONCERNS.
The remaining pages of this TSB article are used to republish past TSB articles pertaining to the clutch system and one new article addressing dash panel cracks at the clutch master cylinder on 1987 and 1988 F-250- 350 Vehicles.