Nickshu

I installed a 997 GT3 brake master cylinder on my 996 today. I had tried several things but none made any significant difference in reducing what is IMO excessive pedal travel and squishiness during hard braking. Slakker here on the forum suggested the 997 GT3 master cylinder as a solution so I put one in today. WOW what a difference. Now the brake pedal feels like a Porsche should.

While this seems to be a very popular upgrade on the Cayman forum I was surprised that little to nothing is posted on the 996 forum about this upgrade, so here are some notes from the install.

The part number for the 997 GT3 M/C is 997-355-910-30 and you can buy a TRW version (OEM supplier) for just over $200 from various suppliers. The cross reference TRW part number is PMN166, but you can still find it under the Porsche part number.

Stock master cylinder size on the 996 is 23.8mm on the C2/early C4 and I think the C4S/Turbo and the 996 GT3 are 25.4mm stock. The 997 GT3 M/C has a 27mm bore.

The 997 GT3 M/C has two M12 outlet ports. Depending on your year and options of your 996 you either have two M12 lines or one M12 (front channel) and one M10 (rear channel) line. My car is a 2003 C2 with PSM and it had one M12 and one M10 line. This means that if you have the M12/M10 setup you have to convert the rear channel to M12 somehow.

Options to deal with the smaller M10 rear channel line is to use and adapter, or make a new line. Making a new M12 line is easy if your car is one that has the PSM booster pump located just below the M/C as the line is only a few inches. My car has the remote mounted PSM booster pump way over on the passengers side so making a new line involves alot of bending so I elected to use an adapter.

I found that there are two main M10/M12 adapters out there on the market, one made by Dorman and one made by AGS. Both have a bubble fitting on the male M12 end, but on the inside M10 female end they are milled to receive an SAE fitting, not a bubble (european) fitting going in.

I found that the Dorman adapter was a bit deeper on the M10 female portion than the AGS, so the Dorman one is the best one to use as it will engage the most threads.

I mounted the adapter under my drill press and using a 21/64" bit modified to cut brass, running at high RPM (2300) I turned the SAE female M10 internal surface into an internally tapered bubble (euro) surface. (You can Google how to file a drill bit for cutting brass w/o gouging.)

This worked out great and everything bolted up really nicely. I tested with very hard pedal pressure (standing on the brakes) several times w/ no leaks.

If you have the skills to do this it makes the install easy and the resulting reduced pedal pressure and travel is excellent!

Photos attached....

lowpue

Thanks for sharing....definitely something I would consider.

Vipertag313

If there ever is a sticky for track-support mods, this needs to be included. Thanks for sharing. I'm right at the cusp of having to address this

moburki

Good info. Thanks Nickshu!

Nickshu

You bet guys! This is a pretty easy mod. Aside from getting the adapter setup it only took me about 1 hour to install and another hour to bleed. And on top of that it's a pretty inexpensive mod. Again I am surprised that there has not been much discussion about this on the 996 forum, the Cayman guys have been all over this upgrade for awhile now.

We had a few inches of snow today...I usually don't drive my car in snow, but couldn't resist....so I took it around the neighborhood, mostly sideways, and gave the PSM and ABS a good workout. No issues noted with the upgraded M/C and any of the PSM/ABS electronics. Just a way better pedal feel and much less travel.

I may have to adjust my adjustable Rennline gas pedal up a bit for heel and toe now. Aaaah champagne problems.

40th steve

Hi,

Any updates on this now that you've had it installed for a while?

Nickshu

Yes... No issues at all. Working great and the pedal feels awesome!

tekkie

Nickshu

I am not sure. I think perhaps the cars with PSM have the smaller M10 rear circuit and the cars without have double M12's. It's easy to pull off the master cylinder cover and take a look, the size of the fittings is obvious under direct visualization.

tekkie

RngTrtl

just out of curiosity, would the 996 turbo or 997 turbo m/c mount up with less machining? I can attest that the 996tt stops like a beast, far better than the 996 n/a that I had did.

Nickshu

I saw threads on another Porsche forum where they needed and adapter on turbos as well. I am thinking that cars with PSM had the smaller rear channel but have not confirmed.

The 996 Turbo and 997 Turbo still has a smaller M/C than the 997 GT3, but yes larger than the N/A models. On the 996 the C4S, Turbo, and GT3 used the same master cylinder. On the 997 the GT3 got a larger master cylinder than the 997 turbo, S, etc models.

For sure there is more to stopping than just the M/C...caliper size, rotor diameter, pad area, etc. My main goal was to get rid of the squishy feel to the stock 996 brake pedal, which for me (and others if you read old posts) I could not get rid of with bleeding, SRF brake fluid, etc. I found it especially bad on the track at high temps. With the 997 GT3 M/C the pedal feels like a Porsche should. Rock solid and short travel.

sweet victory

I'm not a fan of the stock 996 pedal feel. Will definitely consider this! Thank you for sharing!

trendy996

I ended up using a 996 turbo one when I upgraded my front brakes to 390mm and the rears to 350mm on my 99. I had to set up adapters but, I didn't have to do any drilling for it to seal. I found all the proper adapter fittings. I think through ebay. If anyone has a question of if they actually have a 997 gt3 MC you can verify that with those two machined dots on the master cylinder near the reservoir. I'll have to switch to the 997 whenever I come across another one that is cheap. I already have one in my 987.

tekkie