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Working Principles of the 100-4 Master Cylinder

By Norman Nock

Originally published in the Austin-Healey Magazine, September 1996


The 100-4 master cylinder is operated by a push rod connected through brake linkage, directly to the foot pedal. The pushrod on application moves the piston in the cylinder bore and applies pressure to the fluid, which is displaced through the hydraulic pipeline to the wheel cylinders, expanding the wheel cylinder pistons and moving the shoes into contact with the drum. Figure 1 shows the components of the cylinder.

With this cylinder, the main seal or recuperating seal is stationary in the body, a feature unique to 100-4s and a few other British cars only. The plunger moves through it to apply pressure to the fluid. There are four small holes in the plunger which bypass the recuperating seal to allow movement of fluid through the cylinder when the brake is off.

In Figure 2 (right) the brakes are off and the fluid is permitted to flow between the reservoir and the wheel cylinders. Figure 2 (left) shows the brakes being applied, the holes have passed the lip of the seal and pressure is being generated.

The recuperating seal shim allows the system to “breath” between the cylinders and supply tank and also stops any extrusion of the seal through the recuperating drillings when under pressure. The recuperating seal support keeps the seal in place and limits its travel when the pressure is released. The end seal is a ‘wiper’ seal to stop any fluid seeping into the dust cover.

Where a master cylinder is coupled directly to the foot pedal linkage, always ensure that after adjusting the linkage (with the pedal in the full “off” position) there is approximately 1/32” (0.8mm) free play between the push rod and the master cylinder piston.

Dismantling the cylinder

Drain the system of fluid, disconnect the pipe unions and the operating rod from its connection to the foot pedal. Withdraw the securing bolts and remove the cylinder from the chassis.

Clean the exterior of the master cylinder. Unscrew the end cap and remove it complete with the gasket and the seal support. Remove the plunger return spring. Pull back the dust cover and extract the circlip. The push rod can then be removed. Push the plunger together with the end seal from the pressure end of the cylinder, then remove the recuperating seal and the steel shim (or nylon backing – whichever is fitted).


Replace all seals and parts with those contained in the appropriate Girling Service Kit. Clean the remaining parts thoroughly with Girling Cleaning Fluid or unused Castrol-Girling Brake Fluid and place them onto a clean sheet of paper. Examine the plunger and the bore of the cylinder for visible score marks, ridges and corrosion and ch3eck that they are smooth to the touch. If there is the slightest doubt about the condition of these parts, then a new guaranteed master cylinder should be used.

Important: The Service Kit will contain either a nylon backing ring or a steel shim (refer to Figure 1). Discard the nylon-backing ring or steel shim removed from the cylinder and fit the part supplied in the kit.


The rubber seals and plunger should be smeared with un-used Castrol-Girling Brake Fluid immediately before assembly. Fit the end seal to the plunger and insert into the push rod end

of the body, rotating the plunger to ease in the seal. Follow with the push rod assembly and secure with the circlip. Place the nylon-backing ring or steel shim over the plunger and settle in the recuperating seal with the back against the ring.

Place the seal support in position (or fit into the end cap). Fit the gasket and screw on the cap.

Liberally smear the sealing areas of the dust cover with the Rubber Grease supplied in the kit and fit the cover to the cylinder. Fit the rubber band (if applicable) to retain the dust cover on the body and ensure that the opposite end of the cover is located correctly on the push rod collar. If the dustcover fits on the push rod, tighten on the crimping band or bind with wire.

Refit the master cylinder to the chassis in reverse order to removal. When fitting the push rod to the linkage make quite sure that the master cylinder plunger is allowed to return fully, and a little play can be felt between the push rod and the plunger when the brake pedal is off. If the plunger is prevented from returning, pressure builds up and the brakes bind on. The recuperating holes in the plunger are covered and the fluid cannot return.

Fill up the supply tank with Castrol-Girling Brake Fluid and bleed the system.

Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

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