G56 Clutch Master & Slave Cylinder Replacement Guide

  1. Clutch Master & Slave Cylinder Operation
  2. Hydraulic Line Connection Types
  3. Master-Slave Cylinder Troubleshooting & Diagnostics
  4. Master-Slave Cylinder Failure Symptoms
  5. How to Test the Clutch Safety Switch
  6. G56 Master & Slave Cylinder Parts List
  7. Clutch Master-Slave Bleeding Techniques
  8. G56 Master & Slave Cylinder Replacement Procedures

Applicable Models:

2005 - 2010 Dodge Ram 2500. 3500, 4500, 5500
2011 - 2018 Ram Trucks Ram 2500, 3500, 4500, 5500

Applicable Engine(s):

5.9L Cummins Turbodiesel
6.7L Cummins Turbodiesel

Applicable Transmission(s):

Mercedes G56 six speed manual transmission

G56 Clutch Master-Slave Operation

The G56 manual transmission relies on a master-slave clutch actuation system, utilizing hydraulic pressure to disengage the clutch in lieu of a mechanical linkage. Although mechanical linkages remain common in medium and heavy truck classes, hydraulic master-slave systems have long been commonplace in lighter duty applications such as pickup trucks and passenger cars. A hydraulic master-slave setup has several advantages; there are no need for adjustments (self adjusting/self compensating), no complex linkages, and they deliver a smooth and consistent clutch pedal feel since the mechanical advantage is more consistently delivered.

Master-slave systems can be a bit of a headache as they are often difficult to bleed and many cheaper products on the market simply don't last. Many drivers even find that OEM parts have a limited service life, and this simple component can leave a vehicle stranded abruptly and without warning.

The operation of a master-slave hydraulic system is quite simple and they rely on the same principles as a typical hydraulic jack. By taking advantage of the relationship between force and pressure, a small input force can result in a greater output force. The system consists of the clutch master cylinder, clutch slave cylinder, and a hydraulic line connecting the two units.

As the clutch pedal is depressed, the force is transfered from the clutch pedal lever arm to the clutch master cylinder pushrod. As the pushrod travels inwards, a piston pressurizes fluid in the master cylinder cavity. This fluid is transfered through the hydraulic line and into the slave cylinder, where the pressurized fluid acts against the slave cylinder piston, causing the slave cylinder pushrod to move outwards. This outward force acts against a lever that is connected to the clutch throwout bearing. The working fluid (hydraulic fluid) in the G56's master-slave system is DOT 3 brake fluid. Figure 1 below demonstrates how the clutch master-slave system works.

clutch master-slave system operation
Figure 1 - Hydraulic master-slave clutch system operation

Master & Slave Cylinder Line Connections

When replacing the clutch master and slave cylinder as an assembly, there is no need to disconnect the lines from either component; installation is performed as a prefilled, pre-bled assembly. If the clutch master or slave is being replaced independently, the lines will need to be disconnected from the respective component so that the new one can be installed. Note that there are two common types of line connections found on clutch master and slave cylinders. The first style, as seen in figure 2, is a clip type connection. The OEM MOPAR replacement master and slave cylinders utilize this connection. To separate the component from the hose, simply pull the metal retraining clip out of its grooves and then pull the hose out of its port.

clip type line connection
Figure 2 - Clip style clutch master-slave line connection

The second connection type uses a roll pin to secure the hydraulic line to the component, as seen in figure 3 below. An 1/8 or 5/32 punch (as pictured) is used to drive the roll pin out of its bore. Once the pin has been removed, the hose can be pulled out of its port. Although a single roll pin installation is much more common, note that some components use two roll pins to secure the line connection. These can be difficult to remove with the components mounted to the vehicle and we recommend removal of the master-slave assembly for such procedures.

roll pin type line connection
Figure 3 - Roll pin (spring pin) style clutch master-slave line connection

In both instances, the hose is going to be sealed to the component with an o-ring seal or flat washer type gasket (figure 4 below). It is common for the seal to slip off the hose fitting and become lodged in the component port during removal, so always verify that the seal has been removed to avoid double stacking. Generally speaking, these seals can be installed dry. However, lubricating the seal with DOT 3 brake fluid before installation may help form a more reliable connection.

hydraulic line seal
Figure 4 - Clutch master-slave line seal

It's also worth noting that some master cylinders utilize a twist-lock type mounting flange. The flange, the component that mounts to the firewall via two studs, is an independent part that can be separated from the master cylinder. With this style flange, the master cylinder can be removed quickly by pushing it into the firewall and turning approximately a 1/4 turn to release the spring tension after disconnecting the clutch safety switch and clutch pedal pushrod. A visual inspection from under the dash will identify whether-or-not the master cylinder has a twist-lock flange; installation is reverse.

G56 Clutch Master & Slave Cylinder Diagnostics & Troubleshooting

Master-slave systems typically fail abruptly and without warning, although it is possible to develop a leak while maintaining clutch functions for a brief period. The master cylinder fluid reservoir is quite small, thus a leak will quickly allow air to enter the system. The most obvious sign of a failed clutch master or slave cylinder is a condition where the clutch will not disengage and the gear shift level will not engage any gear while the engine is running. You may notice the truck attempt to "creep" as the gear shift is forced against a synchronizer, but it won't engage that corresponding gear (note - accelerated synchro wear will occur from forcing the gear shift lever into a gear under such conditions). While there are mechanical clutch problems that can produce similar conditions, the most likely culprit is the master-slave system.

Symptoms of a Master or Slave Cylinder Failure

• Unable to engage any gear while the engine is running

• Clutch will not disengage when the pedal is depressed

• Clutch pedal goes to the floor, does not return to upper position

• Clutch pedal feels soft or spongy

• Master cylinder, slave cylinder, or hydraulic line leaking

Leaks that are occur around the hose inlet/outlet fittings may be the result of a failed line seal while leaks around the master/slave cylinder pushrods represent a failed or blown out piston seal. Note that it is entirely possible for a master or slave cylinder to experience an internal failure without displaying an external leak. If you are experiencing repeated master-slave failures, you likely have a mechanical clutch problem or severely worn clutch; removal of the transmission is required for inspection of the clutch and its related components.

How to Test the Clutch Safety Switch

If there have been any problems correlating to the clutch safety switch (interlock switch), it is quite easy and convenient to test it when it has been removed from the vehicle. The safety switch is a simple device with a pair of copper terminals oriented at one end and a sliding busbar arranged at the other. One terminal is for reference voltage and the other is the signal return. When the circuit is open, there is no voltage through the signal return wire. A groove on the clutch master cylinder pushrod causes the busbar to slide over the copper terminals, completing the circuit, when the clutch pedal is depressed.

These switches can fail by several methods, but the most common problem is wear in the sliding mechanism that causes excessive clearance between the copper terminals and busbar. More often than not, the operation becomes intermittent before finally ceasing to work. A gunk often forms inside the switch body that is a mixture of grease and fine plastic particles, so cleaning the terminals may alleviate some problems. If you find yourself standing on the clutch pedal or pumping it to get the engine to start, the switch should be tested.

A simple continuity test identifies whether or not the switch is functioning properly. Start by selecting the continuity function on the multimeter and probing the two connector pins (figure 5 below). Since this is a continuity test, it does not matter which probe is touching which connector terminal.

Clutch safety switch connector
Figure 5 - Probing clutch safety (interlock) switch connector with multimeter leads

With the connector probed, actuate the switch by hand (or with the blade of a screwdriver) by extending the sliding mechanism over the copper terminals. In figure 6, the slide is located on the right and the terminals are on the left, therefore the mechanism needs to be slid from right to left. With the switch actuated, there should be continuity between the two connector terminals. If there is no continuity or there is a high resistance reading instead, the switch needs to be replaced.

Clutch safety switch test
Figure 6 - Actuating clutch safety (interlock) switch on a work bench

The clutch safety switch found on G56 equipped trucks is a very common style used on a wide range of vehicles, but the connector may be unique to Dodge/Ram applications. Note that aftermarket versions of the switch do not seem to include the locking plate used to secure it to the clutch master cylinder pushrod; if your locking plate is damaged, you may need to source the OEM MOPAR switch. Napa, SMP, and ATP branded switches did not include the locking plate at time of publishing. Although quite simple, this part is relatively pricey.

G56 Master & Slave Cylinder Parts List

Product Description Part Number(s) Remarks
Master-slave assembly MOPAR 55366325AF (w/ OEM dual mass flywheel only)
Perfection Clutch PF9106 (w/ OEM dual mass flywheel only)
Perfection Clutch PF9064 (w/ single mass flywheel conversion only)
Clutch master cylinder Perfection Clutch 800077 [2]
Clutch slave cylinder Perfection Clutch 900086
Master-slave hydraulic line --- [3]
Master cylinder pushrod bushing MOPAR 4643448 [4]
Clutch safety (interlock) switch MOPAR 53007944AB
Hydraulic fluid Amsoil DOT 3 brake fluid
MOPAR DOT 3 brake fluid

[1] - OEM master and slave cylinders are only sold as a prefilled, pre-bled assembly. Recommend replacing as one assembly if unable to determine which component has failed. Owners whom have converted to a single mass flywheel must use a compatible master-slave system.
[2] - We have found Perfection Clutch master-slave cylinders to be reliable aftermarket alternatives in various applications, including the G56, ZF, and New Venture transmissions.
[3] - At time of publishing, no suitable replacement line was available; if a new line is needed, purchase a master-slave assembly.
[4] - Pushrod eye bushing is not included with all replacement clutch master cylinders and may need to be purchased separately; do not reuse bushing if it appears worn or damaged.
[5] - Amsoil brake fluid is a high performance, long laster alternative to standard fluids. Brake fluid, like diesel fuel, absorbs moisture from air and has a limited shelf life once the seal is broken. Do not use unsealed brake fluid that has been stored for more than 6 months.

Master & Slave Cylinder Bleeding Techniques

Bleeding a master-slave system can be a headache; the hydraulic line is narrow, the total fluid capacity is small, and even a minute amount of air trapped in the system can impede proper function. Whenever feasible, bench bleeding the master-slave system is advisable. Bench bleeding is achieved with all components removed from the vehicle and the hydraulic line properly secured between both components. The master cylinder is then secured in a vice (figure 7) and the slave cylinder placed on or near the floor. The slave cylinder is then pumped by hand while held at a slight angle so that air does not become entrapped at the front of the piston (rear of the slave cylinder higher than the front with the hose port pointing upwards so air can easily escape up the hydraulic line).

clutch master cylinder mounted in vice
Figure 7 - Clutch master cylinder secured in vice for bench bleeding

If the slave cylinder is new then the pushrod will be secured to the housing in the retracted position with a plastic clip (figure 8). This clip is intended to be broken by hydraulic force after the master and slave cylinders are installed and the clutch pedal is depressed for the first time. Its purpose is to allow easy installation of the slave cylinder without having to overcome the spring pressure in the device. Before bench bleeding, unfasten the clip from either side of the slave cylinder housing after pushing the pushrod inwards slightly. After the components are bench bled, reinstall the clip in reverse order.

clutch slave cylinder plastic retaining strap
Figure 8 - Clutch slave cylinder pushrod retaining clip

After pumping slowly 2 to 3 times so that the slave cylinder pushrod travels from its fully extended to fully retracted position, remove the fluid reservoir cap (figure 9 below) and add DOT 3 brake fluid as necessary. If the fluid reservoir level is not maintained and it runs below the master cylinder piston level, air will be pumped back into the hydraulic line/slave cylinder cavity and any progress will have been lost. This process should be repeated 10 to 12 times or until all air is removed. Once it is observed that the fluid level is not changing, the bleeding process should be complete. Never actuate the master cylinder pushrod with the slave cylinder disconnected from the transmission; the slave cylinder piston will be pushed out of its bore. Never pump the slave cylinder with the fluid reservoir cap removed; brake fluid will evacuate the reservoir in a large stream and air will be reintroduced into the system.

clutch master cylinder fluid reservoir
Figure 9 - Clutch master cylinder fluid reservoir

There are additionally two methods of bleeding the system as installed using the bleed screw/orifice integrated into the rear of the slave cylinder (figure 10 below). Gravity bleeding is achieved by removing the fluid reservoir cap, loosening the bleed screw, and maintaining the fluid reservoir level as gravity takes effect. This process is useful in that it only requires one person, but it is often unsuccessful in bleeding the system completely. Air tends to remain trapped in the slave cylinder piston cavity as fluid from the reservoir trickles through the hydraulic line and out the bleed orifice.

clutch slave cylinder bleed screw
Figure 10 - Clutch slave cylinder bleed screw

The more traditional method of bleeding the master-slave system when it is installed on the vehicle requires two people; one to operate the clutch pedal and one to manage the bleed screw. As one person slowly depresses the clutch pedal, another opens the bleed screw momentarily, then closes it before the clutch pedal is allowed to return to its upper position. The bleed screw must be closed before the clutch pedal reaches its fully depressed position and must remain closed while the clutch pedal is returning to its upper position or is resting in the upper position. Air will infiltrate the system if these conditions are not met. The fluid reservoir level must be frequently checked to avoid reintroducing air in the system. Repeat until the clutch pedal feel is appropriate and the clutch is disengaging properly. This procedure can be quite tedious and many cycles are generally required, however it is overwhelmingly effective.

How to Replace the Clutch Master & Slave Cylinder on a G56 Transmission

Note - these procedures outline the removal and installation methods with the master-slave cylinder installed as a single assembly; for disconnecting, connecting, and bleeding techniques, see respective article sections above.

Click any thumbnail to view fullsize, detailed image

clutch slave cylinder location

• With the engine off, place the transmission in gear, set the parking brake, and chock the rear wheels. Disconnect both negative battery cables.

• Locate the clutch slave cylinder on the driver side of the transmission just above the PTO access cover.

• Remove the (2) slave cylinder mounting nuts with a 13 mm socket or box end wrench. Disconnect any clips securing the hydraulic line to the body/frame.

clutch safety (interlock) switch connector

• Locate and disconnect the clutch safety (interlock) switch connector to the driver side of the clutch pedal assembly.

• Remove the clutch safety (interlock) switch from the clutch master cylinder pushrod. We highly recommend checking the switch for wear/damage. Operation of the switch can be tested by running a continuity test across the two connector terminals with the switch mechanism engaged by hand.

clutch master cylinder pushrod

• Remove the clutch master cylinder pushrod eye from the clutch pedal arm pin, then move the pedal forward to access the master cylinder flange studs.

clutch master cylinder mounting flange studs

• Remove the (2) clutch master cylinder stud nuts using a 15 mm deep socket.

clutch master cylinder

• Pull the master cylinder out away from the firewall, then maneuver the slave cylinder and hydraulic line out of the vehicle through the engine compartment.

• Perform any necessary repairs if replacing only the slave or master cylinder, then bench bleed the system.

new clutch master cylinder installed

• Install the new master-slave system assembly by maneuvering the slave cylinder into position through the engine compartment, then installing the master cylinder through the firewall flange.

• Install the master cylinder nuts and tighten snug. Verify the master cylinder fluid level.

• Install the slave cylinder through the bellhousing and tighten both nuts; torque to 16 ft-lbs (23 N-m, 200 in-lbs). Secure the hydraulic line to the body/frame using any supplied clips.

• Test for proper clutch pedal feel and clutch disengagement.

  • The G56 manual transmission employs a hydraulic master-slave clutch actuation system.
  • Bleeding the master and slave cylinder system is best accomplished using the bench bleeding method prior to installing the components.
  • MOPAR only offers the clutch master and slave cylinders as an assembly; the units are not available for purchase separately.
  • The clutch master and slave cylinder system uses DOT 3 brake fluid as its working fluid.