Featured Cars
Research & Articles
Maintenance & Repairs

Reconditioning the Bentley Mk VI brake servo motor
(Applies to all early post-war Rolls-Royce and Bentley cars;
Rolls-Royce Silver Wraith, Silver Dawn, Phantom IV, Bentley Mk VI & R-Type)

Note: The processes described below will refer to the components by the numbers in this diagram, courtesy of the Service Handbook for the Rolls-Royce Silver Wraith, Silver Dawn, Phantom IV, Bentley Mk VI & R-Type

Removal of brake servo from car

Start by removing the engine undershield to expose the servo motor.
I found that it was not necessary to remove all the rods and linkages as described in the manual. The first items that must be removed are the two drag links (44 & 45) that operate the master cylinder from the servo. To remove these rods you first need to remove the bolt (48) from joint J (arrowed above) On the Mk VI it is located near the battery directly under the master cylinder. The picture above was taken looking from the rear and looking towards the front of the car.

Once the two drag linkages are released from joint J you are able to move the two inner servo levers to a position which will enable you to reach the little bolts so that the rods 44 & 45 can be removed from the servo motor. This close-up picture of the servo end of rods 44 & 45 illustrates how it is attached to the servo arms. You need to remove the little bolt, then the locking plate which will in turn release the pin.

Once the lower rod (44) is removed simply rotate the servo to bring the upper servo arm into a suitable position so that the upper rod (45) can be removed in the same way. Now disconnect rod G by removing pin F from lever R (arrowed) This is the rod that activates the rear brakes. This joint is located towards the rear of the chassis.

Use a screwdriver to lever the joint towards the front in order to allow space for the pin to be removed. You now need to remove rod G from the outer servo lever (joint 36). In order to reach the outer servo lever linkage it needs to be levered towards the front of the chassis. This is achieved by depressing the brake pedal. Either find a helper, or make a plan, as I did, pictured above...

Rod G is connected to the outer-most servo lever (arrowed white) With the brake pedal depressed the lever is forced into reach. Remove the split pin, but leave the rod retaining pin in for now. Also remove the split pin from the rod H next to it (arrowed red). Note that the pin holding rod H was incorrectly fitted on this car. The split pin on rod H should be facing the servo so that it does not touch rod G. In order to separate the two servo levers enough to remove the two rod retaining pins you need to first release the lock nut (arrowed white), and then loosen the adjuster nut (arrowed red)...

...using a socket on the lock nut and a spanner holding the adjuster nut. Once the nuts are loose unscrew them away from the two servo levers which will enable you to separate the arms just enough to remove the pins to release the rods.

Loosen the servo retaining setscrew (arrowed)... ...and withdraw. The servo can now be carefully removed from the three driving pins in the gearbox.

Disassembling the brake servo

The brake servo removed from the car and on the bench.
Remove the protector ring from the servo housing starting at spring and gently working your way around. Treat it gently as it is made from very soft and fragile material.

The driving plate sub-assembly can now be separated from the rest of the servo unit. If it appears stuck you may have to work it loose with a screwdriver or similar tool because some repairers were in the habit of gluing them together. Be careful not to damage the spring plate (rear cover) as it is made from very soft and fragile material. Remove the driving plate from the spring plate (rear cover).

Clamp the servo in a vice as illustrated above using wood blocks or something similar so that the servo shaft is not damaged by the vice.
Remove the locking nut.

Remove the adjusting nut. Remove the washer.

The outer lever should now be removed from the shaft complete with it's fitted ball bearing. However, in this case the ball bearing has obviously failed, because the outer cover had separated from the bearing housing... ...revealing the mess inside. This confirms why the brakes were binding. The failed bearing resulted in friction between the shaft and the outer lever, which activates the rear brakes.

The bearing was seized solid on the servo shaft, so it required a lot of penetrating oil...
...and a fair amount of force to move it...

...until finally the two levers could be removed.
The two levers. On the left the outer (servo actuating) lever, and on the right the inner (servo operating) lever. Be careful not to lose the three balls inside the inner lever.

Remove the race.
Remove the race cover.

Remove the inner and outer brake actuating levers. Note that there is a washer in-between the two levers. In this case it is a thin, flat washer, but you may have the later modification which is a "wavy" washer. The purpose of this washer is to prevent the two arms from sticking together.
Remove the ball race.

Remove the felt washer and cover.
Finally remove the pressure plate from the servo shaft.

The brake servo is now disassembled.

Reconditioning the brake servo

I had great difficulty removing the servo from the gearbox, and this was the reason why. Judging from the gooey gunk on the servo shaft, it would appear that the servo was glued to the driving pins on the gearbox.
Just by way of demonstration, this is the rear of the driving plate sub-assembly.

If you need to remove the friction lining you must first "slacken" the spring plate. To do this you simply strike the centre of the spring plate as illustrated..
...which will pop up the spring plate...

...allowing you to turn the plate to expose the rivets.
I was lucky in that the friction lining was still in good shape, requiring only light sanding to clean it up and bring it back to life.

This photograph illustrates the difference the sanding made. On the left you can see the oil-stained lining, and on the right the clean lining.
On the outer face of the pressure plate there are two holes. I'm not sure why they are there, perhaps they are drain holes, but the problem is that they allow oil and water to enter the brake servo, rendering it useless.

In order to rectify this it is common practice to seal the two holes off with whatever you prefer to use. Some people make metal inserts, some use rubber plugs and whatever else does the job. Mine had neat little wooden plugs, which indicates that an informed person worked on this servo in the past. I had to re-install one of the plugs.
The plugs are, and should be, a tight fit. I made up a press using my vice to press the plug back into the hole.

The oil retaining felt washer should be replaced while everything is apart. It fits into the retaining washer pictured above it.
I had a batch of seals made up locally, so if you are reading this and need this seal, come fetch one!

The servo operating inner lever has a round sleeve which holds the three balls in place. This sleeve was slightly damaged during attempts to remove the seized outer lever bearing from the servo setscrew. I removed the sleeve from the inner lever so that it can be cleaned and repaired.
The sleeve is a tight fit, so it had to be pressed back onto the inner lever.

This photograph shows the sleeve on the inner lever.
Carefully repairing the damaged sleeve.

The inner sleeve of the lever was also slightly damaged, and had to be filed smooth. Checking for correct operation by replacing the three balls inside the inner lever, and then inserting the outer lever "teeth" into the inner lever. The outer lever must operate smoothly on the three balls when twisted anti-clockwise, which will result in the two levers been pushed away from each other. This simulates the operation of the levers.

The seized bearing in the outer lever has to be replaced.
I used a new modern bearing.

However, as you will see from the picture above, the new bearing has a flatter profile than the original. The original is shaped to fit inside the outer (servo actuating) lever.

I removed the shaped cover from the old bearing...
...and placed it inside the shaped "cup" to serve as a spacer for the new bearing.

The pressure plate has a smooth surface that presses against the friction lining which activates the brake servo (almost like a clutch). This surface can become "glazed" and very smooth, which makes the servo less effective.
Place the pressure plate in a vice and sand the friction surface to flatten it and remove the "glaze" so that it has a better "bite" when pressed against the friction lining.

Reassembling the brake servo

Place the servo shaft in a vice.
Put the pressure plate in position.

Put the new felt washer inside the retaining washer and coat it with grease.
Put it in position with the felt washer downwards.

Put the ball race cover in place. You may have to gently tap it into position with a soft hammer.
Coat the ball bearing cage with grease making sure to squeeze it trough the balls. This also forces out any old grease that may still be in there.

Place the bearing inside the cover.
Put the inner lever in position.

Coat the washer with grease and put it in position on top of the inner lever.
Put the outer lever in position.

Place the spacer inside the thrust bearing cover. Make sure that the side with the groove is upwards.
It may have to be pressed in position. I used a vice and a socket to press it home.

Fit the cover assembly to the servo shaft.
Grease the thrust race and place it inside the cover.

Put the inner lever with the three balls onto the shaft. Put grease on the little ramps on which the balls run. and position each ball to be at the base of it's ramp...

...which will allow the outer lever to slot nicely into position on top of it. Make sure the two arms line up.
Put the servo shaft washer in position...

...followed by the two nuts. Position the outer nut at the end of the servo shaft for now. This will prevent damage to the shaft end when maneuvering the servo back onto the car.
Now turn the assembly upside-down and mount it in the vice.

Put the driving plate sub-assembly onto the pressure plate. Turn the driving plate until the holes line up with the holes in the servo shaft. This is very important because when you fit the servo back on to the gearbox the servo shaft must engage with the three driving pins on the gearbox. You will have to line the holes up by "feel" while fitting the servo to the gearbox.

Before fitting the spring plate to the servo make sure that it is in good condition, and not damaged or dented. This is not merely a dust cover, it forms part of the operation of the servo. Test it as illustrated above. By pushing down on the centre it must have a definite spring action.
Place the spring plate over the driving plate...

...and seal the edge with electrical tape. This will ensure that water, oil and dirt stays out of the servo assembly.
Fold the electrical tape over the edge to ensure a good seal.

Fit the protector ring..
...and the servo is ready to go back on the car.

Fitting the brake servo

Clean up the area around the three driving pins...
...and very gently and carefully maneuver the servo unit into position. Position the driving plate holes onto the three driving pins, and then turn the servo shaft until it's three holes also line up and the whole assembly can be pushed firmly onto the driving pins.

While still holding the servo in position, fit the servo retaining setscrew and tighten it firmly. Re-fit rod H onto the inner lever, with the pin as illustrated above...

...and then fit rod G to the outer-most servo lever. Note that the back, flat ends of the two pins must face each other so that the two levers do not jam during operation. Swing the upper (inner) lever down and re-connect rod 45.

Push the upper lever back into position...  ...and then re-connect rod 44 to the lower (outer) lever.

Re-assemble joint J by first inserting the shaft...
...and two spacers onto the two drag links (44 & 45)

Fit the bolt...
...and tighten the nut.

Finally, re-connect rod G by inserting pin F into lever R. This is the rod that activates the rear brakes. This joint is located towards the rear of the chassis.

Adjusting the brake servo
With the rear brakes properly adjusted, put the rear of the vehicle on axle stands. Using the brake adjuster, lock the left rear wheel so that it cannot turn.

Slacken off the lock nut to leave enough room to get a spanner onto the adjuster nut.
Ask a helper to spin the right rear wheel (or use a walking stick if you are alone, as I did) while slowly turning the adjuster nut tighter. At some point the upper and lower brake actuating levers will move, pulling the the two drag links (44 & 45) that operate the master cylinder from the servo. The moment you see movement of the two levers, back the adjuster nut off two flats. The servo should now be correctly adjusted.

Tighten the lock nut making sure that the adjuster nut is not moved, or you will lose the correct setting. And that completes the reconditioning of the brake servo. 

 Road test the car before re-fitting the engine undershield and re-adjust if necessary.
Return to Maintenance index page