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Join Date: Sep 2000
Location: sun valley ca 91352
Posts: 63
Repairing the Hydraulic brake booster

I am adding a how to do DIY brake booster repair and bleeding procedure for the ABS/ABD system. I could not find any good info online to correct the problem I was having with my braking system on this car. My hard brake pedal has now been corrected doing what is listed below.

To begin with, I bled the accumulator repeatedly, thinking it was at fault. Bleeding the brake booster system requires having the battery connected normally, and the ignition key set in position 2 or 3. This way, when the accumulator is bled, the pressure switch will trigger the pump to turn on when the pressure drops as you open the bleeder to dispense fluid.

At first a lot of foam was dispensed into my catch can. After 5 or 6 catch can fillings, the pump sounded smooth when it pumped fluid. It there is air in the lines before the pump, or air in the pump, the pump will sound like it is cavitateing. When this procedure was finished, I determined the pump and accumulator, and pressure switch, were working fine.

A 9mm wrench is required, along with a suitable catch can, in this case I used a mightyvac bleeding container connected to the nipple.

After completing this procedure, I decided the booster was the real problem causing my hard brake pedal. To repair the booster, you will need to remove the master cylinder. Repairing the booster can be done without removing the booster from the car, leaving it attached to the pedal assembly. You will not need to dismantle any of the interior parts of the brake pedal assembly. The below pic shows the booster and master cylinder assembly as it is when removed from the car. I have 3 complete braking systems, so I am showing individual parts to help you better understand what you will be doing.

You should follow this procedure when doing this job.
1. disconnect the battery terminal, either positive or negative, or both. This will prevent the pump from turning on when you bleed the accumulator.
2. Bleed the accumulator as stated in the above text.
3. Drain the brake fluid container by carefully prying up the plastic lines from the master cylinder.
Make sure to clean off the rubber grommets that hold the plastic lines in place on the master cylinder. Some spray brake cleaner and pressured air will work to do this. You are going to spill fluid doing this no matter what you do to prevent it. Just stuff a good size rag under the master before removing the lines. You can catch most of the fluid in a clean container as it drains from the fluid container.

4. Remove the 2 steel lines from the master cylinder. The rags will catch what little escapes from these lines. When you are finished repairing the booster, you will not need to re bleed the whole brake system. All you will need to do is reattach and tighten these two lines to the master then, where these same two lines connect to the ABS/ABD pump/solenoid assembly, loosen them, allowing fluid to flow through the master cylinder, pushing the air trapped at the top of the lines where they attach to the master, and then out of same lines where you loosened them at the ABS/ABD assembly, and onto your rags.
See pics of the ABS/ABD assembly at the end of this guide.

5. Remove the master cylinder exposing the front of the booster assembly, then remove the three bolts that attach the front side of the booster, then pull the front piece off, being careful not to loose the two small black orings shown in the below pics. You will lose a few ounces of fluid when you pull it off.

At this point you will need to get a few tools together as shown below:

The 24 inch pvc pipe is used to push the brake pedal inside the car using the power seat to move the pedal forward as you set the depth of the plunger inside the booster.
Old 06-19-2011, 03:28 PM
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Join Date: Sep 2000
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6. Measuring from the step in the booster housing, set depth of the brake rod plunger to 17mm +- 3mm, as shown in the below pic, using the butt end of a caliper similar to the one shown in the above tools pic. Adjust and hold the plunder by placing the plastic pvc pipe between the bottom front of the drivers seat, and the brake pedal. My seat is 8 way adjustable, so I simply moved the seat forward and backward until I got the desired depth set correctly. I reattached the battery temporarily for this purpose, leaving the key in the OFF position.

7. Now that the plunger is properly held at or near the correct depth by the plastic pipe, it's time to remove the hydraulic piston assembly for cleaning. The two boosters I fixed had the same problem. The small piston gets stuck in its bore due to gummy old brake fluid.

7a. First remove the 27mm bolt from the top of the booster casting. This part holds the link between the brake rod plunger, and the booster piston assembly. I used a long 1/2" extension with a universal joint on its end to get to the top of the booster to remove this part. Put the long extension with the 27mm socket and the U joint down to the back of the booster, and loosen and remove the part.

Old 06-19-2011, 03:30 PM
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7b. Next unscrew the hydraulic piston assembly. This requires shaping a tool to fit in the end of the piston assembly. The second pic shows how I ground the corners of a pry bar off so that it would fit the tapered slot of the piston assembly. Before unscrewing this set of parts, make sure to clean around the bore so that the green rubber seal is not damaged by some bits of crud that may have accumulated over the years.

Once you get the piston assembly out, you may need to use a small drift to move the plunger that caps the outer end of the piston bore. This small cap is held in place by a clip, similar to a piston pin clip. The cap must be tapped down in it's bore in order to loosen the stuck hydraulic piston that butts up against it. It is the piston that needs to be removed. You are removing the piston, but do not remove the plug. Once you move the plug a few mm, the piston should be loose.

Now you can remove the piston from its bore and clean it all out with brake cleaner etc. You may need to tap the plug back up into the bore against the retaining clip. After cleaning the piston and bore, the piston should move freely therein. The tolerance of the bore to piston is about as tight as it gets. It barely fits. All of these parts are the highest quality I have seen on any car.

Reassemble in reverse order adding brake fluid to the necessary surfaces for lubrication. The green oring may require careful attention when reinstalling the piston assembly into the housing. Orings tend to expand over the years, so you may need to carefully push the assembly into the bore being careful to get the green oring back into the hole without pinching it.

Putting this repair guide together took longer than doing the repair. I hope reading it does not take even longer.
Old 06-19-2011, 03:31 PM
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dan79brooklyn's Avatar
Join Date: Nov 2004
Location: Okayama, Japan
Posts: 1,016
Nicely done.
Current Drivers - MKIV GTI & MB E320 Wagon
(saving for another 911...maybe a 996)
EX - 1993 Porsche 911 Carrera 2
EX - 1979 Porsche 911SC TARGA
EX - 1976 BMW 2002
Old 06-23-2011, 06:25 AM
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sdm952's Avatar
Join Date: Sep 2003
Location: santa barbara
Posts: 44
993 c4 booster repair problem

awesome article. after i performed this booster repair procedure i have a new problem. as soon as the booster pump is switched on the brake pedal gets sucked to the flow slowly and the brakes are locked up. any ideas??
Old 08-30-2012, 05:32 AM
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Any clues to the answer for why the booster would suck the pedal to the floor?
Old 09-03-2012, 04:48 AM
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Great article, should be added to Porsche manual. The pedal getting sucked in is most likely improper assembly, tear it apart and rebuild again or hydraulic problem during reassembly! One thing is does anyone know where to source all the o-rings on the piston mainly the green one! It was ripped when I removed the piston bore.
Old 10-01-2017, 09:35 AM
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