Clutch Hydraulics

Standard Transmission & Gear, Inc.

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Clutch Hydraulics
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Standard Transmission & Gear is now a proud distributor of AMS Automotive’s transmission products.


Many late-model vehicles actuate the clutch system using a hydraulic clutch master cylinder and slave cylinder, rather than mechanical linkage. When the clutch pedal is depressed, the pushrod on the master cylinder displaces fluid from the cylinder, through a tube, to the slave cylinder. In most instances, the piston attached to the slave cylinder moves a release fork, attached to a release bearing, in order to disengage the clutch.

On other systems, the slave cylinder piston pushes forward against a release bearing to disengage the clutch. Hydraulic systems are designed to reduce pedal effort, and to simplify installation by more directly connecting the pedal to the clutch. They also eliminate the need for failure-prone items like return springs and cable mechanisms. Because hydraulic systems have built-in pedal travel already factored in, periodic adjustment is usually not required.

Problems generally arise when either the master cylinder or slave cylinder leaks hydraulic fluid, draws air, or fails to maintain adequate pressure required to disengage the clutch. If one of the components is damaged or leaks, it should be replaced before new clutch components are installed. On high mileage vehicles, it is also advisable to replace both components, even if only one of the components appears damaged. That’s because both components are designed with the same expected service life.

The AMS clutch hydraulics program provides industry-leading coverage on nearly 100% of all applications. Each sku is cataloged alongside the other components required for a complete clutch system replacement. Application specific tech bulletins and proper bleeding procedures are included with many sku’s. In addition, we use an “exact fit” approach, rather than relying on “will fit” aftermarket consolidations. This provides assurance of proper fit with the other components, and eliminates the need to search multiple catalogs for the necessary components.

Hydraulic Slave Cylinder

Clutch Slave Cylinders

The function of a slave cylinder is primarily controlled by the master cylinder. Slave cylinders are divided into two primary categories: Concentric slave cylinder (CSC) and external slave cylinder.

Concentric Slave Cylinder

The CSC is mounted inside the bell housing where the release bearing retainer is normally attached. This unique design offers the following benefits: Elimination of the clutch fork and related hardware, precise release bearing travel with better alignment geometry, and closer fit to the transmission which increases flexibility in passenger compartment design. A CSS uses a large coil spring to maintain constant contact between the bearing face and diaphragm spring fingers. Therefore, no “lost travel” is required between the release bearing and diaphragm spring before the clutch pressure plate begins to disengage.

Many newer style plastic body CSC designs include plastic shipping straps attached to the plastic bushing and to the body. These straps compress the overall length of the unit during installation, which means the pushrod does not need to be compressed in order to clear the release lever. If the hydraulic system has been properly bled, the straps will break when the first stroke of the clutch pedal moves the pushrod forward. Like other suppliers, AMS offers the CSC with the angular contact bearing already attached. However, AMS was the first supplier to offer popular CSC sku’s without the release bearing. Because the clutch kit contains the release bearing, this eliminates the duplicate cost of a release bearing when both the CSC and clutch kit are installed.

(No-bearing) Concentric Slave Cylinder

Since their introduction in light truck applications in the late 1980’s, clutch release systems containing a concentric slave cylinder (CSC) have become the hydraulic system of choice in domestic applications. Not only are they reliable and easy to troubleshoot, they also eliminate the need for other parts such as the release fork, quill, and pivot linkage. In many instances, the CSC is equipped with a replaceable angular contact release bearing. Because the thrust face surface of this bearing maintains constant contact with the pressure plate diaphragm spring, the bearing may fail before the remainder of the CSC. When installing a new clutch kit in a CSC equipped application, it’s important to always inspect the hydraulic system. If it is operating properly, simply replace the old release bearing with a new release bearing (contained in the clutch kit), along with the other components found in the kit. However, if the clutch will not release and/or the hydraulic system is leaking or will not bleed properly, a new CSC should be installed, along with a new master cylinder and clutch kit. In this instance, the technician is now forced to purchase two release bearings, one that’s contained in the kit and one that’s already attached to the new CSC.

In order to eliminate the duplicate cost of sourcing two release bearings, AMS offers the industry’s first and most comprehensive lineup of no-bearing (NB) options. Many of these NB options are among our most popular slave cylinders. What’s more, they offer a significant price reduction versus the same slave cylinder with a release bearing. Although the vehicle owner ultimately pays the price for unnecessary parts, many of the extra release bearings end up being returned to the jobber or WD. Because no paper trail exists for the sale of the extra bearing, the distributor may end up issuing an inflated credit for the bearing. All NB options are cataloged using the standard slave cylinder part number, followed by an NB suffix (i.e. S0710NB)

External Slave Cylinder

As the name implies, external slave cylinders are normally mounted on the outside of the transmission bell housing. They consist of few moving parts other than a piston, internal spring, and pushrod. A rubber boot is usually used to seal contaminants away from the moving parts. The release bearing used in this system always maintains preloaded contact with the diaphragm spring. The external end of the pushrod is normally tipped with plastic, mushroom-shaped, bushing that serves to reduce noise and eliminate surface wear on the clutch fork pushrod tip. The plastic bushing is also important because it helps determine the overall length of the rod. This exact length is very critical in the overall function of the hydraulic system.

Hydraulic Master Cylinder

Clutch Master Cylinders

The clutch master cylinder is directly connected to the clutch pedal and serves as the portion of the hydraulic system that generates hydraulic pressure. Master cylinders are divided into two primary categories based on the internal mechanism used to generate hydraulic pressure: seal over port systems and center-feed type systems.

Seal over port system

The seal over port system uses a push rod attached to the clutch pedal to produce piston movement in the master cylinder. When the piston moves, seals attached to the piston move forward. The primary seal passes over a small port in the wall of the cylinder, which allows fluid flow to and from the reservoir. Once the primary seal passes over the port, the fixed portion of fluid contained between the master and slave cylinder is moved toward the slave cylinder. This results in disengagement of the clutch. One variation of the seal over port is a plunger type master cylinder. Rather than using the attached seals that move with the pistons, the piston instead moves through the seals.

Center-feed type system

The center-feed type system uses small hooks attached to the nose of the piston. When the clutch pedal is not depressed, these hooks engage a flange on the valve stem. A circular rubber seal on the opposite end of the valve stem is held away from the master cylinder body. This allows fluid flow to and from the reservoir. When the clutch pedal is depressed, the hooks no longer hold the valve stem and it is pushed to the bottom of the master cylinder body. The reservoir is now separated from the fixed fluid, contained between the master and slave cylinder. As the clutch pedal is further depressed and the master cylinder piston moves even further, fluid is forced toward the slave cylinder, resulting in the disengagement of the clutch. Master cylinders are also subdivided into two additional categories: Integral reservoir systems and remote reservoir systems. The fluid reservoir of an integral reservoir system is part of the master cylinder body. The fluid reservoir of a remote reservoir system is attached to the master cylinder body via a rubber hose. The use of integral or remote reservoirs is dictated by the available room in the engine compartment. Both integral and remote reservoir types are used in seal over port and center feed type systems.

Hydraulics Pre-filled

Pre-Filled Hydraulic Systems

Late model hydraulic systems require a variety of complex bleed procedures. Obtaining this information is challenging, and properly bleeding these systems is even more difficult. The bleed procedure on a dry system can take up to an hour or more. To resolve these problems, AMS offers the industry’s first and most comprehensive lineup of pre-filled clutch hydraulics, available as a pre-filled system or a pre-filled master cylinder. Each system is pre-filled with hydraulic fluid and pre-bled, eliminating the need to perform this often-difficult task when the components are installed on the vehicle. When compared to the installation of dry components, shop time and the risk of warranty are substantially reduced.

Pre-Filled Systems

This system includes a master cylinder and slave cylinder, connected by the hydraulic line. No additional components are required, however, the pre-filled system may also include features such as a pulsation damper, heat shield, or neutral safety switch. These added components aren’t generally included with the individual master or slave cylinder. The design of these components makes conventional bleeding difficult and time consuming. For example, if the built-in pedal pulsation damper becomes contaminated, the system becomes nearly impossible to bleed. In addition, it is especially difficult to remove air from the damper. The pre-filled system eliminates these difficulties.

Pre-Filled Master Cylinders

This system includes a master cylinder connected to the hydraulic line (includes male quick disconnect). The system must be used with the existing O.E.M. slave cylinder or with a dry slave cylinder purchased separately. This system is designed for applications that include a Concentric Slave Cylinder (CSC) inside the bell housing. In addition, the master cylinder is typically mounted at an angle that makes conventional bleeding difficult and time consuming. After installation of the clutch and pre-filled master cylinder, a two-minute procedure to fill the CSC is all that is required. By comparison, the Ford-approved bleed procedure on a dry system can take up to an hour or more. The quick disconnect is the vulnerable component in this system. It is prone to leak due to component deterioration, and the female portion (attached to the CSC) or male portion is routinely broken during removal because the proper removal tool was not used. Rather than installing a complete pre-filled system, replacing that portion of the quick disconnect that is damaged (master cylinder and hydraulic line or the CSC) reduces the overall cost.