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GT3 master cylinder ugprade for early 996s & Boxsters

Old 07-11-2019, 07:51 PM
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Default GT3 master cylinder ugprade for early 996s & Boxsters

Finally managed to sort this out. Net/net is you can convert your master cylinder to the GT3 part if you have a 996 or Boxster made before Sept. 13, 1999 but it will require simultaneous conversion to the updated brake booster, brake pushrod mechanism, and firewall seal parts. Documenting here for the benefit of others' future reference.

Parts needed:

Your choice of one of these brake boosters with the updated design:
2003 996 C2 Brake Booster (TRW or Porsche Genuine options): 996-355-923-00
2004 996 GT3 Brake Booster (Porsche Genuine only): 996-355-923-90

And one each of these:
997 GT3 master cylinder (TRW or Porsche Genuine options): 997-355-910-30
Firewall seal update set: 000 043 204 66
Updated brake push rod: 996 423 165 05
Threaded bushing: 996 423 307 00
Pin: 996 423 308 00

Tools needed:
1/4” ratchet with extension
3/8” ratchet with extension
1/2” ratchet

Torque wrenches covering values 7.5 ft/lb, 17 ft/lb, and 36 ft/lb

T45 Torx bit (for the long brake booster bolts)

H5 Hex bit (for intermediate piece positioning bolt)

10mm “mini” combo wrench (for the small nut that holds the two pushrods together)
16mm combo wrench (you will need two 16mm wrenches for the brake pedal to pushrod bushing and pin removal)
17mm combo wrench
19mm combo wrench

6mm 1/4” socket (or flathead screwdriver to tighten the bellows clamp)
13mm 1/4” socket
16mm 3/8” socket
22mm 1/2” socket

11mm flarenut wrench for hydraulic unit brake line connections
13mm flarenut wrench for master cylinder brake line connections

Vernier calipers

Small carpenter square (or vernier calipers & straightedge) to verify brake pedal height to throttle measurement

Rubber lubricant or tire mounting paste. I used Super Lube Silicone Lubricating Grease that I had laying around.

Tips/potential hangups:
  1. Save yourself the wasted time and frustration and just remove the ABS hydraulic unit. You will not get the old booster out with it in place. Also, if you have PSM which I didn’t, you will need to remove the PSM booster pump as well.
  2. Same goes for the intermediate piece (#11 on the parts diagram). You will need it fully out of the way to get the new firewall seal pieces in place.
  3. The most tedious/difficult part of the job for you will likely be setting the updated firewall seal pieces (7.2 & 7.1) in place.
    1. Make sure the sealing ring (7.2) is set evenly and fully into the cutout in the firewall.
    2. Pushing the connecting piece (7.1) into the sealing ring is the hardest part. Get the intermediate piece completely removed and out of the way, lubricate the two pieces and use all of your body weight and leverage from the front of the car to push it in. I needed a lot of leverage, a good double handed push and all of my 214 lbs. to get this seated. Unless you have World’s Strongest Man levels of strength, you’re not going to push it in with one hand from the side of the car.
  4. Make sure you note the torque value for the bushing and pin that hold the brake pedal and pushrod together. While unlikely given the design, having it snap and fall out from being over torqued when you stand on the brakes with all your weight to panic stop on the street or at Turn 1 at COTA would mean a very bad day… The indicated value is 7.5 lb/ft.
  5. Go for a test drive in a controlled environment afterward to make sure everything is functioning correctly. Then come back in, pop the hood, and check for any leaks at any of the hydraulic line connections, bolt/nut torques, etc.
  6. I actually did not set the brake pedal height per the spec in the instructions and left the adjustment of the inner pushrod on the master cylinder where it was given that the new mc requires less pedal stroke. I may adjust this in the future based on track testing and will report back about that after the car has been tested in a controlled environment at high brake pressures.
  7. Everything else is per the attached instructions.

In total, I’d rate this 3.5 out of 5 wrenches on the Pelican scale. This is a pretty messy job with the inevitable brake fluid mess and requires a fair bit of disassembly/reassembly, so if you’re unsure you should leave it to a pro. Given you’re messing with the heart of the braking system, which I’m always extra cautious about working on anyway, remember doing it wrong could result in serious injury (or worse) to you or someone else if you get it wrong. Silly I have to say this, but I'm obviously not responsible or liable for either and I'm posting this for educational purposes only.

Feel free to ask any questions here or DM me and I'll be glad to help!

Nearly disassembled but ABS hydraulic unit and intermediate piece still need to be fully removed.

New firewall seal and updated brake pushrod being fitted.

Verifying clearances and test fitting before completing reinstallation.

Fully reinstalled and brake fluid topped up ready to bleed.

Updated firewall seal and bellows fitted.
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Old 07-11-2019, 08:41 PM
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Thanks for the update! As far as I understand, an updated brake booster and push rod is not required to use the 997 GT3 MC. Were you having issues with just the 997 GT3 MC?
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Old 07-11-2019, 08:51 PM
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It depends on when your car was made. Porsche did a mid-model year cutover in Sept of 1999 to an updated booster, pushrod, and firewall seal design. If your car has the older parts the GT3 MC is not a direct bolt on and you will need to update the others in order to make it work.
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Old 07-12-2019, 08:10 AM
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This needs to be a sticky. I have a March 1998 build and this is a mod I will likely do in the future.
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