| The car used in this technical article is Bob Tindel's 1983 911SC, but other models with the vacuum brake booster are similar. Bob's Disclaimer Correctness of this repair is critical to the safe operation of your car. If this repair is done improperly, you could be fatally injured or killed (after all, the brakes are only one of many things that can stop your car...) Air Conditioner Repair Reveals Brake Master Cylinder Leak First, a little story on how I found that the brake master cylinder on my car was defective. During a recent trip, the ambient temperature was unseasonably warm, so I turned on the air conditioner, but the blower didn't work. In troubleshooting this, I first checked the AC fuse and relay on the fuse panel in the luggage boot. Both were good. In further checking, I found that the wire from the AC compressor clutch had rubbed against the pulley, wearing through the insulation. I repaired this fault, but still no joy. Previously, I had upgraded the AC evaporator fan relay to the later style that has an external 15-amp fuse. This relay is located in the "smuggler's box" in the floor of the boot. To access the smuggler's box, pull out the carpet in the bottom of the boot. The lid of the box is then visible. The fuse was indeed blown. After it was replaced, the AC worked fine. However, upon opening the smuggler's box, I noticed a small amount of liquid in the groove around the edge (Hmmmm-the only liquids in this part of the car are windshield washer fluid and brake fluid). The liquid was brake fluid, coming from the brake master cylinder junction with the brake booster. This is where the fluid leaks out when the seals in the master cylinder have failed. To find this defect earlier, check the bottom of the brake booster for leakage periodically. Checking the Brake Booster for Correct Functioning Before disassembly, it is a good idea to check the booster to ensure that it functions properly, so that it can be corrected at the same time as the master cylinder if it is defective. Here is how to make sure it works right: 1. With the engine stopped, depress and release the brake pedal several times to remove any vacuum retained in the booster. 2. Hold down the brake pedal with medium effort, and start the engine. 3. If the brake booster is functioning properly, the brake pedal will give slightly underfoot as the booster takes effect.
Brake Master Cylinder and Brake Booster Assembly Removal CAUTION: Exercise care when working with brake fluid near any painted surfaces-spilled brake fluid can cause serious paint damage! This is one of those jobs that looks harder than it really is. The brake master cylinder, brake booster, and brake master cylinder are removed as an assembly. Then any necessary repairs or replacement can be done on the bench. If the master cylinder is leaky, I recommend replacing it rather than rebuilding. The cost differential is not great, and a replacement unit provides all new parts, including brake light switches, correctly assembled. The first step is to disconnect the operating rod. To access this rod, remove the driver's side carpet, and then the driver's footboard. One 10mm nut at the bottom center, and a locating tab at the top hold the footboard in place. It is easier to remove the footboard if you peel off the rubber pedal covers first-that way, you don't have to disturb the adjustment of the rubber clutch pedal stop on the footboard. (Figure 1) The throttle rod just unsnaps from the back of the accelerator pedal After the footboard is out, remove the lock pin at the top end of the operating rod. On some models, the pin has an integral retainer that just snaps onto the operating rod clevis. Other models have a separate retainer. Snap off the retainer, and remove the pin. (Figure 2) Next, remove the brake master cylinder mounting bolt, which is located inside the cabin on the luggage boot floor plate. (Also Figure 2) You don't need to disconnect anything else under the footboard. The rest of the work will be done in the luggage boot. Remove the carpet from the bottom of the boot, and remove the fiberboard dash cover at the rear of the boot. The cover is held in place by four 10mm cap screws, two on the top and two on the bottom. (Figure 3) There is a flat washer and a lock washer on each screw. Be careful not to drop anything in this part of the boot, because it will probably fall into an irretrievable place. If your car has cruise control, remove the rectangular cruise control "brain" by unplugging the cable from the bottom. Then remove the brain itself (two Phillips screws), and the brain mounting bracket (two 10mm cap screws). (Figure 4) This gives a little more room to work. Disconnect the paper/wire AC duct hose from its inboard end at the plastic plenum. Gently bend it up out of the way, to get better access to the master cylinder. You may also wish to remove the plastic duct hose above the paper one. (Figure 5) Remove as much of the brake fluid as possible from the master cylinder reservoir with a rubber bulb syringe. Put some rags under the master cylinder to catch any spills. This will make the job a bit less messy. Disconnect the overflow hose from the reservoir. Disconnect the electrical connectors from both brake light switches. (Figure 6) Loosen the brake booster vacuum hose clamp, and disconnect the hose. (Figure 7) Disconnect both brake lines from the master cylinder. (Figure 8) Remove the bolt from the brake booster brace. (Figure 9) Remove the four hex nuts from the base of the brake booster. (Also Figure 9) Carefully lift out the brake booster/master cylinder/reservoir assembly. Again, use caution not to spill brake fluid on any painted surfaces. Torque Value for Brake System Fasteners Don't let the terms "Foot-pounds (Ft/lbs)" and "Newton-meters (Nm)" confuse you. There are complicated formulae to convert these two, but for the real world the relationship is 4 Nm equals 3 Ft/lbs (i.e., 100 Nm equals 75 Ft/lbs). The easiest conversion is simply to multiply the torque in Nm times .75, so 25 Nm equals about 19 Ft/lbs. Here are the sizes and torque values for the fasteners involved in the repair: 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)
Replacing the Master Cylinder The next part of this procedure can be done on the bench. Drain the as much of the remaining brake fluid as possible from the master cylinder reservoir. Disconnect the two rubber brake fluid hoses from the reservoir to the top of the master cylinder. Remove the two plastic fittings from the top of the old master cylinder, and insert them into the rubber fittings on the top of the new master cylinder. Remove the two hex nuts holding the master cylinder to the booster, and separate these two units. (Figure 10) When you finish bench bleeding the master cylinder, you may want to reinsert the rubber plugs temporarily into their openings to make reassembly with the brake booster cleaner. Install the rubber O-ring on the master cylinder, and reassemble it to the brake booster. Torque the two hex nuts to 25 Nm. These are called "staybolts", and if they are stretched or broken, replacement of the brake booster is required, so use caution when tightening them. (Figure 11) Reconnect the reservoir hoses to the master cylinder. Reinstall the master cylinder/brake booster assembly, and reconnect the brake light connections, brake lines, and vacuum hose. From inside the car, reconnect the brake operating rod and reinstall the master cylinder mounting bolt. NOTE: On some cars, the two rubber hoses from the brake master cylinder reservoir to the master cylinder trap the cable for the ventilation system control valve. If so, you can loosen the tiny nut that holds the end of the cable, unsnap the cable housing retaining clip, and pull the cable free, so the master cylinder can be removed. The tedious part is putting the clamp back in place after the master cylinder is reinstalled. If you drop anything in this location, it can be difficult to recover. To prevent loss of this clip, tie a piece of dental floss to it, and snap it in place with a pair of long-reach needle-nose pliers. Bleed the Brakes Using your favorite method, bleed the brakes to purge all of the air that was introduced into the system during this repair. It is good practice to bleed the entire brake system several times, starting with the caliper furthest away from the master cylinder (RR) and ending with the closest (LF). This is a good time to completely flush the brake system of the old brake fluid. During bleeding, keep the master cylinder reservoir topped off to prevent more air from being introduced into the system. Final Inspection After you finish bleeding the brakes, you should have a firm brake pedal that engages at about thirty to fifty percent of its travel. Check the tightness of all of the fasteners, including the operating rod and master cylinder mounting bolt, and check the brake lines, master cylinder and booster for any brake fluid leaks. If all is correct, reinstall the dash cover and luggage boot carpet. Reinstall the footboard and carpets in the cabin. Carefully road test the car at low speed, away from other traffic. You may wish to bleed the brakes again after a few days of driving to ensure that all trapped air bubbles have been removed from the brake system. Reset Brake Warning Light After brake repairs are completed, the brake warning light on the dashboard may be on. It can be reset by briefly disconnecting the battery ground strap. Self-Congratulation Give yourself a big pat on the back! You just saved a significant amount of money. The dealer estimate for this repair is $354. | 
