Vacuum Boost

Technical Information and Restoration Parts for the 928

Brake Booster

Theory of Operation - The booster is designed to take a vacuum not pressure. When you press on the pedal the vacuum combined with a spring is use to increase or multiply the force that is applied to the brake master cylinder. A vacuum by nature is a lack of air. The lower pressure causes the atmospheric pressure up to 14 pounds per square inch to be exerted in an attempt to equalize the pressure differential. So the way a vacuum can be converted to work is by allowing air to displace the vacuum. This is usually done by a check valve that allows a vacuum to accumulate in the bladder of the booster and causes a positive pressure to escape.

Engine vacuum varies all over the place in the normal operation of an internal combustion engine. Hooking a vacuum gauge to the intake manifold shows a decrease in vacuum on acceleration and an increase on deceleration. Relying on the engine to supply vacuum to all of the actuators that are on the 928 and the brake booster though the normal operation cycle would create flat spots. Older cars used vacuum to drive the windshield wipers. When accelerating the wiper would stall and then run very fast on deceleration

Connected directly to the engine the actuators for the heating system and the power brake booster would not work on acceleration. To get around this problem a check valve is inserted into the system that allows the vacuum in the booster to accumulate on deceleration and not to be used by the engine during acceleration.

In this fashion the car is really getting free energy as the vacuum is accumulated and stored for later use. Vacuum actuators are cheaper to build than electric motors and in theory would be more reliable.

A reserve tank is used to store vacuum and is connected with a check valve between the engine and the booster. Unlike air pressure that can be compressed into a liquid or a solid, a vacuum can only be a –14.7 pounds per square inch. No matter how much air you take away you can only get to zero. Of course there are limits to how good a vacuum one can get and how much integrity the system can handle – but that is beyond this discussion.

All materials are permutable and with age materials deteriorate causing a vacuum to equalize with the atmosphere. After shutting off the engine, the booster will loose vacuum slowly. Depending on condition the system it should hold a vacuum a couple of hours or even over night or a couple of days.

Diagnosis - Diagnosing a bad brake booster is a simple job.

1. Turn off he engine and press down on the brake pedal several times until it is difficult to push.
2. Run the engine for a few minutes. While the engine is running press on the brake pedal. It should operate smoothly. If the pedal remains stiff then the power booster system is not working
3. Shut off the engine and let the car sit from ten minutes. Press on the brake pedal. If the pedal is stiff or can not be pumped three times with boost then the system is leaking vacuum.

Vacuum Pump - An alternative method of checking the booster requires the use of a vacuum pump. A handheld vacuum pump can be used to evacuate the booster and isolate the check valve as the problem. It can also be used to measure the leak down rate.

Quick Test - A quick and dirty way of checking the booster is to hook a hose to its inlet and blowing into it. (Do not use compressed air as it can damage the booster). When blowing into the booster it should not leak.

Replacement- Replacing the booster is tricky but can be down by most with modest mechanical skills providing that the steps are followed carefully.

1. Remove the master cylinder. The reservoir just pulls off. Turn it upside down quickly and you will not loose much fluid. Be careful to cover the fender and all painted parts as brake fluid can easily remove paint. There is special 7/16" or 11 MM wrench that is made for removing brake fittings. It is an open end wrench that goes 3/4 of the way around the fitting and has a longer handle for better leverage.. This prevents fitting from getting stripped and knuckles from getting scrapped. I highly recommend this wrench.

2. You may have to remove the brake cross over or equalizer depending on the year. It also helps to remove the air cleaner to make some room in the area.

3. For the next step you will need vice grips. Push down and lock the brake pedal with a stick. On the outside of the booster use the vice grips to hold push rod in its extended position.

4. On the inside of the car remove the clip on the pin holding the push rod to the pedal and push the pin back freeing the push rod from the pedal. Then push the pin back into the pedal arm in such a way that pin comes in contact with the front of the brass portion of the push rod. This allows the pedal to increase the throw or travel on the booster. Again push the pedal to the floor and lock it with a stick again. On the outside of the car adjust the vice grips to hold the push rod as far out of the booster as possible.

5. What this operation does is to move the collar that goes through the firewall on the booster flush with the side of the booster. Unless this collar is flush with the booster it is impossible to remove. Not enough clearance.

6. Remove the brass portion of the push rod on the inside of the car by releasing the lock nut and unthreading it from the push rod on the booster. Remove the rubber boot on the inside of the booster.

7. Remove the rubber shield over the four bolts holding the booster to the firewall if it is still in place and remove the four nuts holding the booster in place. A long extension helps with this operation.

8. At this point the booster should slide out between the engine and fender. Be very careful to get the vice grip on tight and not knock it off while trying to remove the booster. If the vice grip come off then you will have to put the booster back in and use the pedal to compress it again before being able to remove it. I was able to do this with one nut holding the booster and using the point on the pedal without attaching so the second and third times around go quicker.

Installation - To install the booster is just the reverse process. You will have to use a clamp or vice to collapse the booster and attach the vice grip before starting the operation. Take note or mark the adjustment on the inside push rod so you can get it close to adjustment on installation. If the rod is adjusted too tight it will cause the brakes to drag.

If you have a hand vacuum suggested above use it to bleed the brakes. If you have a clutch then take this opportunity to replace the blue feed hose. They deteriorate over time and will just fail or rot out. You should also inspect the flex hose coming out of the clutch master for age as again it is easier to replace with the booster out..