Replacing the Master Cylinder

Replacing the Master Cylinder
Paradise Garage

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Pedal to the metal is only a good thing when it's the right pedal involved.

If you're ever sitting at a stoplight and notice that the brake pedal starts creeping toward the floor, that's a sure sign the seals in the master cylinder are about to give up. You'd better park the car and deal with the problem while it's early.

Some people like to buy a master cylinder rebuilding kit and take a shot at fixing it themselves; others like to save money by buying rebuilt units. Frankly, with a little shopping it's easy to get a new replacement master cylinder at a very reasonable price. Considering how dangerous a master cylinder failure is, there's no way we'd settle for anything less. So we contacted our friends at Coventry West and ordered up a new Lucas/Girling master cylinder (Lucas #PMK181 or Jaguar #CAC-1583) to install on the Paradise Garage 1985 Jaguar XJ-S.

This is where the master cylinder tries to hide.

Installing a new master cylinder is incredibly easy on this car. Open the bonnet and use a turkey baster to suck the brake fluid reservoir dry. Put the used fluid into a glass jar.

Disconnect the reservoir lines at the master cylinder. The clamps require a flathead screwdriver. No doubt there will still be a small amount of fluid left in the system -- be ready to catch it with that glass jar. You don't want it taking the paint off the inner fender. By the way, this is a great time to upgrade the reservoir.

Jam some rags under the master cylinder and disconnect the metal brake lines on the side of the cylinder. The rearmost fitting uses a 7/16" wrench; the foremost fitting uses 1/2". They might dribble a little; that's what the rags are for.

Remove the two nuts holding the master cylinder to the power brake servo. They're 17mm and they have lock washers under them.

Remove the old master cylinder.

Be sure the new cylinder gets the old one's black rubber seal, like this.

Compare it to the new one. Gently turn the reservoir nipples on the new one to match the alignment on the old one. Also, note that the old one has a rubber seal where it mates with the servo. The new unit does not come with this seal, so note its alignment and transfer it to the new cylinder.

Take this opportunity to clean everything you can reach that you normally can't. Pay particular attention to the inside of the servo bore.

Remove the cap covering the end of the master cylinder and reattach it to the servo.

Remove the brake line plugs and reattach the brake lines.

Remove the nipple caps and reattach the reservoir lines.

Flush your entire brake system. Replacing the master cylinder will have introduced air to the system, and anyway, your brakes almost certainly needed it by now.

That's it! Go out there and enjoy your newfound stopping power.

Never a glorious part, the lowly master cylinder doesn't get the attention it deserves. It is one of the few parts on your car where its failure can kill you. When you get that pedal-drop warning sign, don't make excuses -- take care of the problem immediately or stop driving the car until you have time to deal with it. Jaguars just don't look good with a Honda stuck to their grille.