This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
Without a doubt, your brakes are one of the most important systems on the car. The heart of the brake system is the master cylinder, which controls the hydraulic pressure of the entire system. Unfortunately, over many years, the master cylinder has a tendency to wear out and leak. The leakage can occur internally or externally, resulting in a weakened braking system. If you have any problems with your brakes, and you think that it's related to the master cylinder, you should probably replace it.
On the early cars without power brakes, the master cylinder is located underneath the car covered by a large panel often called the belly pan. To gain access to the master cylinder, you need to remove this panel. The master cylinder is bolted to the pedal cluster, with the chassis of the car sandwiched in-between. Access to the master cylinder is usually pretty good if you have the front of the car raised off of the ground. See Project 40 for more details.
The cars equipped with power brakes have the master cylinder located in the front trunk compartment. Prior to removing the master cylinder, it's a wise idea to confirm that your brake vacuum booster is working properly. You can usually tell during normal driving, however, Bob Tindel of Pelican Parts suggests an alternative method of testing. Press the brake pedal a few times when the engine of the car is off. Holding down the brake pedal with a light touch, start the engine. The brake pedal will give way slightly under your foot, indicating that the vacuum booster is pulling on the pedal. If it doesn't, then you might want to consider replacing the vacuum booster, or checking the vacuum hoses that supply the booster.
In order to remove the master cylinder on the power brake cars, you need to first disconnect the actuation rod and mounting nut from inside the cockpit of the car. Remove the floorboard (see Project 40 for additional details) and remove the small pin that holds the actuating rod to the master cylinder. In addition, there is a small nut that holds the master cylinder to the chassis. Remove this nut before you move back into the front luggage compartment.
In the front of the car, you now need to start removing a few things. Start with the carpet and remove the front cover to the air blower (See Project 69 for more details). It's a wise idea to hang on tight to all nuts, bolts and tools during this entire procedure, as it is difficult to retrieve items that are dropped in the front trunk. Remove the fresh air hoses that get in your way, and also remove the silver, rectangular cruise control brain if your car is so equipped. Now disconnect the vacuum lines from the brake booster and the overflow hose from the reservoir.
Whether your car has power or manual brakes, the removal of the brake lines is very important. As with the installation of new flexible brake lines, it is very important not to strip out the fittings on the lines. You should always use an 11mm flare-nut wrench to remove the fittings from the master cylinder. See Project 48 for more details. It's also a wise idea to spray the area with some WD-40 or other lubricant if the lines seem to be heavily corroded. This will usually be the case more with the early cars because the master cylinder is located underneath the car, and is more susceptible to the elements.
Before you remove the master cylinder, make sure that you siphon off as much brake fluid as you possibly can using a turkey baster or other applicable tool. Disconnect the hoses that connect the master cylinder to the reservoir. Remember that you want to avoid spilling any brake fluid on your paint. On the manual brake cars, the master cylinder is held in using two nuts attached to the pedal cluster. On the power brake cars, the vacuum booster holds in the master cylinder using four nuts mounted to the floor of the front trunk.
After you have the master cylinder removed, you can take it over to your workbench. Unbolt it from the vacuum booster and separate the two units. If your new master cylinder is missing any small hoses or fittings, then transfer them from the old one. Reattach the new master cylinder to the brake booster being careful not to torque the nuts past the value of 25 Nm (18.4 ft-lbs). If you over-tighten these bolts, you may permanently damage your brake booster, which is a very expensive part to replace. Make sure that you install the small o-ring into the master cylinder before you attach it to the brake booster.
When reassembling the master cylinder in your car, it's important to use the proper torque values. Follow the specifications in this table:
|Location||Tool Size||(Thread)||Torque Nm||(Ft/lbs)|
|Master cylinder to brake booster||13mm||(M8)||25||(19)|
|Brake booster console to trunk floor||13mm||(M8)||25||(19)|
|Brace strut to console||17mm||(M10)||46||(35)|
|Brake lines to master cylinder||11mm||(M10 X 1)||14||(11)|
|Bleeder screws in calipers (front)||9mm||3||(2)|
|Bleeder screws in calipers (rear)||7mm||3||(2)|
When the master cylinder is reinstalled, it's time to bleed your brake system. You may want to dry bleed the master cylinder on the bench in order to prime it before you start the install. For more information on bleeding your brakes, see Pelican Technical Article: Bleeding Brakes. Following the bleeding of the brakes, reassemble all the surrounding parts in the trunk that you have disassembled, and make sure that everything is tightened. Reinstall all the carpets and the floorboard in the cockpit.
When you are ready to drive the car, make sure that you test the brakes beforehand. Don't drive near other cars, and prepare to use the emergency brake if necessary. It's probably a wise idea to bleed the brakes again a few days after you install the new master cylinder to make sure that you have gotten all of the air out of the brake system. If the emergency brake warning lamp lights up on the dashboard, then you might need to either manually reset the switch on the master cylinder (for manual brake cars) or briefly remove the battery ground strap on the power brake cars.
Right above the foot pedals on cars with power brakes, is the connection to the master cylinder and the retaining nut. Remove the actuating lever by removing the small pin that attaches it to the pedal cluster (shown by white arrow). Also shown in this photo is the mounting bolt for the master cylinder assembly (green arrow).
This photo affords us a great look at the master cylinder in power-assisted 911s. The two main brake lines are the metal ones exiting diagonally out of the master cylinder. The fluid is fed from the reservoir to the top of the master cylinder via the cloth braided rubber hose. The two electrical switches that exit out of the side of the master cylinder are used to detect pressure drops in the system.
Make sure that you use the appropriate torque values when tightening the connections to the master cylinder. Over tightening the vacuum booster can damage it internally. This is also a good time to bench-bleed the master cylinder. This process basically fills the master cylinder with fluid and primes prior to installation in the car. Bench bleeding the master cylinder can save a little time later on, but it can also get a little messy.