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my vacuum brake booster conversion (long)
Posted by: a-talk (2292) on 2013-06-06 21:51:41

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After much research, purchasing several parts I didn't use, and many late nights scratching my head, I finally completed my vacuum brake booster conversion. I found just a few people on the 'net that have done this conversion on the C3, and lots more that have done this on a C4. Last year 'loxxrider' did this conversion on his 20v turbo, but I'm not sure what he used for a PS pump afterwards. See his article for details: loxxrider vacuum boost conversion

THE GOAL:
Get rid of the hydraulic brake booster system and replace it with a vacuum boost system. Use Audi parts where possible. Retain the original PS pump to keep it simple.

STEP 1: Remove the master cylinder
I was able to unbolt it from the booster and undo the brake lines, but keep it attached otherwise (I had spare bleeder screws lying around to seal up the two feeds to the ABS pump).

STEP 2: Remove the hydraulic booster
As others have noted, you need to remove the four nuts holding the booster to the firewall. Access is from underneath the dash after removing the under dash panel. Plan on a very long 1/4" extension and a flashlight, and taping the inside of your socket so you don't loose any retaining nuts.

STEP 3: Remove hood alarm switch
The top of the vacuum booster hits it on installation. I plan on making a little jumper so I can arm the alarm again.

STEP 4: Install a vacuum booster from a B3
Use the clevis and pin from the B3 booster (the original clevis and pin are too small). I purchased a new seal between the booster and MC (rubber O-ring) to be sure that the new booster would hold vacuum reliably. Some have claimed to use a C4 10" diameter booster, but I test fitted one and wouldn't have been able to fit it without moving or modifying quite a bit of stuff.

STEP 5: Reinstall the brake master cylinder
Remove the spacer that was between the MC and the old booster, then bolt it up. You may have to loosen the brake lines at the ABS pump so you can rotate them to reach, as the MC sits a bit further back now. I had to *slightly* bend the brake lines as well to reach. Then bleed the brakes.

STEP 6: Disable brake booster portion of PS pump
Remove the pistons under the two X caps nearest the front of the pump and reinstall the X caps. Then just thread a bolt with a crush washer seal into the now unused hole that fed the brake bomb.

STEP 7: Plug the unused fittings on the PS reservoir
I had extra hose and wanted the PS level sensor to still work, so I chose to use my existing reservoir. Some short hose, a few bolts and some hose clamps sealed up the unused fittings. A C2 or B2 reservoir may have worked as a bolt-in replacement, but you wouldn't have a working level sensor.

STEP 8: Run a vacuum feed to the booster
I just used a tee fitting into the back of the IM, and used generic booster plastic vacuum line from the local AutoBuddy. The vacuum check valve in the booster I chose was also a generic unit, as it fit the plastic vacuum line better than the Audi check valve.

STEP 9: Lastly, remove the brake bomb and associated lines
You don't need them anymore! :)

On to the photos:

B3 9" booster vs. C4 10" booster
9

Hydraulic booster removed from the car
where the booster used to be

Test-fitting the booster in the car
vacuum booster in the car

Booster is installed!
booster installed 1

Vacuum feed line to booster with generic check valve
booster vacuum line
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