The cylinder is made in two styles ; one with which the fluid reservoir is an integral pan of the cylinder (Fig. 1.), and the other with a threaded inlet port to which is connected the supply tank by a length of pipe (Fig. 2.).
The operating principle of both is identical. When pressure is applied to the pedal the push rod contacts the plunger and pushes it up the bore of the cylinder. Fig. 4. In the first 1/32 inch of movement the centre-valve seal closes the port to the supply tank and as the plunger continues to move up the bore of the cylinder the fluid is forced through the pipe line to the cylinders.

On the return stroke the plunger moves back with the return of the fluid and the final movement of the plunger lifts the valve seal off the seat allowing free flow of fluid between system and tank, Fig. 3.

The cylinder consists of an alloy body with a polished finished bore with an inner assembly which is made up of a push rod, dished washer, circlip, plunger, end seal, plunger seal, spring thimble, plunger return spring, valve spacer, spring washer, valve stem and valve seal, Fig. 3. The open end of the cylinder is protected by a rubber dust cover.

Several variations have been made and they concern mainly the dust cover and plunger. Originally the plunger was fitted with one seal and later a groove was cut in the plunger to accommodate a taper end seal of almost square section. When servicing, the original plain plunger should always be replaced by the later type with groove as illustrated.

Three types of dust cover are in use: (a) a small cup shaped cover, as Fig. 1, (b) a concertina-type cover with a rubber band to seal it on the cylinder and a metal clip to secure it on the push rod, as Fig. 2, and (c) a rubber disc held in position against the open end of the cylinder by a metal cup secured by two or three indentations, as Fig. 5.

Some versions of the cylinder with separate supply tank have two vertical ports but they are being replaced by the cylinder with the inclined outlet port, as Fig. 2. A slight curve in the outlet pipe is needed to accommodate the change to the later type.

Apart from these differences the servicing of the C.V. cylinders is the same.

DISMANTLING

Drain the system of fluid by attaching a rubber tube to a bleed nipple on one of the wheel cylinders and unscrewing the bleed screw a half turn and pumping the fluid into a clean container. Unscrew the pipe nut or nuts on the master cylinder, the fixing bolts, disconnect the push rod and remove the cylinder from the chassis.

Pull back the rubber dust cover and remove the circlip with a pair of long nosed pliers. The push rod and dished washer can then be removed. When the push rod has been removed the plunger with seal attached will then be exposed. Remove the plunger assembly complete. The assembly can be separated by lifting the thimble leaf over the shouldered end of the plunger. Ease the plunger seal off the plunger and remove end seal.

Depress the plunger return spring allowing the valve stem to slide through the keyhole of the thimble thus releasing tension of spring. Detach the valve spacer, taking great care of the spacer spring washer which is located under the valve head. Remove the seal from the valve head.

Examine the bore of the cylinder and if it is not scored or ridged and is smooth to the touch new seals can be fitted with confidence but if there is any doubt of the condition of the bore then a new cylinder should be fitted.

If the old seals are loose on the plunger and appear oversize, suspect mineral oil contamination. This is the greatest single cause of cylinder failure and sufficient care is not taken to avoid it. Natural rubber seals are very susceptible and minute amounts can cause serious swelling in time. If it is suspected change all seals and hoses and flush out the system with clean brake fluid.

ASSEMBLING

From the appropriate Service Kit replace the valve seal so that the flat side is correctly seated on the valve head. The spring washer should then be located with dome side against the underside of the valve head, see Fig. 6, and held in position by the valve spacer, the legs of which face towards the valve seal. Replace the plunger return spring centrally on the spacer, insert the thimble into the spring and depress until the valve stem engages through the keyhole of the thimble making sure the stem is correctly located in the centre of the thimble. Check that the spring is still central on the spacer. Refit new plunger seal on to the plunger with flat of seal seated against the face of plunger, and a new back seal with lip of seal facing plunger seal. Insert the reduced end of plunger into the thimble until the thimble leaf engages under the shoulder of the plunger. Press home the thimble leaf, Fig. 7 .

Smear the plunger well with Girling Red Rubber Grease and insert the assembly into the bore of the cylinder, valve end first, easing the plunger seal lips in the bore. Replace the push rod with the dished side of washer under the spherical head into the cylinder followed by the circlip which engages into the groove machined in the cylinder body.

Fit the new rubber dust cover, refit the cylinder to the chassis in the appropriate manner, smearing with Wakefield-Girling Red Rubber Grease where it contacts the cylinder and push rod.

Connect up the pipes, charge with Wakefield/Girling Crimson brake and clutch fluid and bleed the system in the manner described in section 4. With this type of cylinder air can best be removed by pressing the foot pedal slowly to its limit and releasing smartly and repeating. If the cylinder is of the type in Figure 1, a filter with a recess as shown in the drawing Fig. 8. should be fitted. When correctly topped up the base of the filter should be just awash.

When connecting the push rod it is essential to preserve the 1/32 inch gap between the push rod and plunger when the foot pedal is fully returned. In those instances where the pedal returns until checked by the cylinder there is no danger but where a separate pedal stop is fitted care must be observed.