Can that hole be brazed over or plugged with a screw or bolt? Does anyone know what is behind the brass fitting?

 

It’s not threaded. I considered JB-welding it shut but seemed sketchy for the part responsible for, you know, stopping you. I couldn’t sort out on any of the diagrams I found what that fitting actually does, and prodding around with a bunch of brake module shops around the world, none of them touch the hydraulic half. Only solution I could find was… replace!

 

I've also found that the Mazda RX-8 actually uses the latest version of that hydraulic block and "looks" like it should work as well, and they are much much cheaper to find used. They have some different kind of caps/plugs on those relief holes on the block.

 

Nice find! Wish I knew this a couple years ago! Have you confirmed mapping off the hard line holes is 1:1? E.g. right rear comes out of the same port on both, etc.?

 

Haven't gotten one physically in my hands, but from pictures, it looks identical, complete with the pressure switch accomodation. This is a pic of the RX-8 unit, and you can see Bosch did something with those leak prone ports on the side.

 

Looks like they map the same indeed. Here’s my original block from the 2001 996tt. “V” in German is for “vorne” as in front, and “H” is “hinteres” for rear.

Though the Mazda block has those two Frankenstein black plugs to either side. Not sure what those are, but the Porsche block doesn’t have that.

 

Right, other pics of the newer RX-8 block those "Frankenstein" plugs, lol, align with where the leaky ports are on ours.

 

Thought I'd pos another update.

TL;DR: further ABS/PSM gremlins went away after applying electrical contact cleaner to any fuses related to the braking system.

After replacing the hydraulic block and bleeding, still had issues at the next couple track days (Thunderhill, Laguna). Things would be fine for half the session, and then mid-sesh the dreaded ABS and PSM dashboard lights would fire up, as in the first post above. One time this happened right before the hard brake at turn 11 at Laguna... brakes locked the wheels up without ABS, flat-topping my brand new Michelin Pilot Cup 2's 😣

Pulled the codes from the ABS module with the Durametric cable (careful - they clear if you turn off the engine) to find code 4276: Valve Supply Voltage:



I replaced the battery with a fresh one hoping for an easy fix, but to no avail. Suspected the alternator or its voltage regulator, but that was chooching at a healthy 14.6V or so at ~3k RPM, so that couldn't have been it. I noticed this tended to happen when braking fast and in heavy cornering on the track, leading me to hypothesize a simultaneous switching voltage problem. Suspicion was: PSM engaging during cornering and braking causes a lot of activity on multiple solenoid valves simultaneously, drawing a lot of current to the module. If any wiring along the path had increased resistance, that would manifest the a voltage drop reported by the module.

So next came finding the source of the increased resistance. The grounding wire from the ABS module to the nearest chassis GND had a fairly oxidized copper ring screwed down to the chassis with a nut on a stud. I scratched it clean with a file and re-seated the nut. Another couple laps round Laguna and the problem returned.

Peeping the 996tt wiring diagram, I saw 3x fuses that seemed to have something to do with PSM/ABS by name:


Fuse B9 - CU ABS PSM
Fuse E9 - PSM
Fuse E10 - PSM

Pulled all three, and they looked fine. Shuffled them around with other fuses of same amperage, then grabbed some electrical contact cleaner from Autozone and gave the spades of the fuses a good squirt before reinserting. Theory was: these guys haven't been touched in 20 years and 165k miles and likely oxidized, leading to increased DC resistance.

I'm pleased to report that for 3x track days since, I've had no more ABS or PSM Christmas trees light up! Posted my personal bests, and for the first time in nearly a decade, enjoyed track days without any car problems! Here's to many more 🍻

Hopefully this helps someone!

 

Thanks a lot for sharing this very interesting information....

 

Great documentation krizzle! After installing the new ABS hydraulic/pump unit, you mentioned bleeding process with “pre-charge pump” hot-wired. Is this the pump installed on the right of the frunk that has 3 ports: one coming from the brake booster, 2nd one coming from brake master cylinder and the 3rd going to ABS hydraulic?

Theoretically on a new ABS hydraulic/pump unit can also trap air bubbles, is there any need to activate the pump of the ABS hydraulic unit in the bleeding process?

Also if you could recommend a manual that has all the diagrams of the wiring and routing of brake/hydraulic lines that would be helpful.

 

Nah, the pre-charge pump is the big aluminum barrel looking thing below the reservoir and master cylinder. It has a soft line from the reservoir, two hard lines to the ABS valve block, and a single 2-pin power connector going to it:



Yep, when changing out the valve block or opening any hard lines, it's possible air bubbles get into the labyrinth. So hot-wiring that pre-charge pump (if you don't have access to the PWIS system) is the way I got enough fluid flow to eke out all the bubbles. Positive pressure Motive bleeder or vacuum bleeding at the caliper nipples didn't really get much volume of fluid moving fast, and I surmised that bubbles would go back up via gravity faster than the fluid can spew them out. The ABS pump runs *hard* and seemed to work perfectly in one out of one tries. Note that on the Durametric documentation for the pre-charge activation switch (which, remarkably, doesn't work as documented - didn't seem to do anything at all!) there's a warning about not running it for more than about 15 seconds, because it's not really meant for continuous use and might overheat (see post #12 again). It really squirts fluid hard and fast, bubbles with it. For normal bleeding where none of the hydraulic lines are disconnected, there should be no need to run the pre-charge pump like this.

As for diagrams... googling "996 turbo wiring diagram" leads to this great rennkit PDF. Page 24 has the pinout of the PSM and pre-charge pump connector, page 40 the fuse table, and page 43 the wire color code lookup table. I couldn't find a better diagram than in post #11 for the hydraulic lines, but it's fairly easy to map this with your own eyeballs while looking at the unit in the frunk. Post #21 has the mapping of rear/front/left/right lines coming out of the block.

Good luck!

 

Thanks krizzle for the charge pump photo! It is the unit I thought to be pre-charge pump, however for some non-turbo 996 it is located in the right side of the frunk and for others it is missing altogether. I saw photos of 996 that supposedly (including the brake booster) get vaccum pressure from the engine without a dedicated charge pump. I wounder why dedicated pre-charge pump was introduced in some design, perhaps for finer grain control on the pressure. I am thinking of buying a 996 Bentley manual, I wonder if the manual explains the system functionality.

 

Not all 996s have PSM (stability management). The pre-charge pump is just for PSM equipped ones I believe. Aside from the 959 unobtanium, the introductory 2001 996 Turbo was the first-ever Porsche to get PSM, if I recall. Most 996.2 models after ~2002 had PSM and hence the pre-charge pump. The hydraulic block has its own pump motor too, used for ABS which I believe all but the GT3 996s have. All 996 models have a vacuum line to the big round brake booster to which the master cylinder attaches, as that’s the main force multiplier for power brakes.

 

Just installed part # 99635575561 (supposedly from 2004 996 with PSM) hydraulic block into 2001 996 hydraulic controller (which originally came with part # 99635575554). I am yet to test the system, but the hydraulic block and controller is a 1:1 fit.

One of the output lines from failed hydraulic block had to be cut off due to stubbornly stuck fitting. So I need to find a replacement hydraulic line. Could not find the part number of the output hydraulic lines from ABS unit to the attachment to the body. Anyone diagram of the hydraulic lines would be helpful. Is it safe to reuse the cut off line with bends and flared fitting? I think it is an F-Type convex flare.

 

Happy to post that after 2 months of diagnosing the issue to finding parts and looking through diagram to understand 996 braking system, finally she is up and running and stopping properly!

Big thanks to this post by krizzle that made the repair possibility of DYI with the idea of original ABS controller and alternate hydraulic unit, which not only made the fix easier, without spending thousands on new ABS pump unit.