Thought I'd share a success story of fixing a leaky ABS/PSM hydraulic unit on my 2001 996 Turbo. Might save someone some cash like it did for me!

TL;DR: you can mix-and-match metal valve blocks from cheaper units with the plastic controller/coil block from your original 996TT unit to replace a leaky one. With an evening's work, I saved ~$3k over a new unit.

On the last heat of an otherwise successful track day, my brake pedal hit the dreaded floor but the car still coasted. Thought I boiled my fresh brake fluid, but when I opened the frunk, I found a swimming pool of RBF600! This was followed by some nasty warnings:


I pulled the plastic covers from the frunk to reveal the brake system, all covered in fluid. It was hard to tell which of the pump, master cylinder, and hydraulic unit was leaking as they were all sprayed in fluid. I taped some paper towels and went for another brake-heavy drive with a topped-off reservoir to narrow down the leaking component. I could go some ~50 miles on a highway before the fluid leaked to the point of warnings again.

Paper towels helped narrow down the leak

It was clear that the hydraulic unit, 996-355-955-51, was leaking:


It looked like it was coming from a pressure relief valve that was hidden by an easily-removable plastic cap.

I set off researching replacements, and learned a bunch of info from these forums, mechanics, dealers, and hydraulics repair specialists around the world:

• The 996-355-955-51 unit is particular to the Turbo. Can't trivially interchange from any other PSM-enabled 996 trim.
• The -51 version had been superseded 6x since 2001 by -57, -58, -59, -63, and -CX which is the current one
• New units earlier than -CX are virtually impossible to find, and the cheapest -CX was $2.8k from Sonnen Porsche
• New units are hardly ever "new" - they're usually core-replaced refurb ones.
• The only used -51 variant I could find was $800 from a Swedish salvage shop.
• Controllers in the unit are tied to your central ECU and a new one would need reprogramming
• No shops I could find would consider fixing the valve block - not even specialists. All would go about with replacements.

I was about to spring $800 on that Swedish unit until Tyler from modulemaster.com (not a shameless plug - genuinely helpful guy!) educated me on some hydraulic unit basics. You can separate the unit's metal valve block and plastic controller half and mix-and-match the two halves! He helped me find a 996-355-755-47 used unit on eBay for $100, which comes from base 996 Carrera and is much more plentiful in the wild. Even though there were different part numbers, from the eBay images it appeared the metal half was identical to my Turbo's unit. Worth the gamble!

To remove the unit, I used various flare wrenches to remove the hard metal brake lines. The pressure sensor in the back and cable connector come off easily. Make sure to have many towels handy for the dripping fluid!


It was useful to tape up the contacts of the removed plug to avoid damage or fluid entry. Took about 2 hours to set the unit free. I took many pictures along the way with my phone to ensure I wouldn't confuse which line went where.

I was able to split my original Turbo 996-355-955-51 into two halves:

• Metal valve block Bosch P/N 0 265 225 029
• Plastic controller Bosch P/N 0 265 225 029

Only ~6x torx screws keep the halves together. The valve unit is fully sealed so there's no danger of damaging the hydraulic circuit inside, and there's a nice rubber seal that keeps dust out. The entire unit is almost solid-state, with coils in the controller half activating sealed solenoid valves in the metal half. Pretty neat stuff inside:


With the unit removed, I was able to replace the metal half with the donor Carrerra 996-355-755-47 unit's block P/N 0 265 255 027. The fit was perfect, and by keeping my original controller half, I could avoid reprogramming the part numbers in the ECU.

Using the pictures I took pre-removal, I worked all brake lines back into place and hand-tightened them about as strongly as I could. I pulled the carpeting and soaked up the fluid mess in the frunk:


A thorough wash with isopropyl alcohol revealed that even after some ~3 weeks of sitting there, the RBF600 didn't strip the paint! Washing the carpets out was easy - brake fluid is water soluble! A quick hosing-down and drying overnight did the trick.

Now came the tricky part. Since opening the hydraulic system like this introduced many bubbles, a simple fluid refill + pedal / Motive pressure bleed wouldn't suffice. At a mechanic, they would hook up the $20k PIWIS system to turn on the ABS pump and do a thorough flush. All mechanics wanted some ~$300 for this job, which matched the price of the Durametric cable I finally sprung for.

The bleeding procedure was the normal one (search any of the forums), except for each wheel I'd stop mid-bleed and turn on the ABS pump relay in the Durametric tool to run the ABS unit for about 30 seconds. Either it was too quiet to hear it operating, or it's actually closed-loop controlled with the unit's pressure sensor, but I could never hear or feel it turn on to push more fluid. Whether it was totally necessary to use the Durametric tool, I still don't know. But I have it now!

I repeated the bleed twice for good measure, going through some 4 quarts of RBF600.

Results were excellent - brake felt even stiffer and more responsive than before, and I've had no problems after some 5k miles since.

Hope this helps others facing similar problems!

 

Wow this is good information! Thanks for sharing!!!! This leaking hydraulic valve was on the metal portion of the ABS unit and it was leaking underside of the metal portion? I am confuse, sorry, I am trying to understand exactly where the leak was coming from ;-)

 

Yup, the metal block itself leaked from what appeared to be a pressure relief valve on the underside.

 

Ok thanks, I never saw the underside of the ABS unit. I've found some pictures on Ebay....

 

 

 

 

There looks to be two variants for the 996: one with PSM hat has 6 hard lines out of the valve block, and one without PSM and only ABS that has 4. As long as you get the right variant, the block and controller appear interchangeable and line up well.

I’m in the San Francisco Bay Area!

 

 

Welp, 3 years, 16k miles and about 3-4 track days later, same exact thing happened on same exact track 😖

I figured I’d just bite the bullet and buy a new 996-355-955-CX unit. I was sad to find that while they cost $2.5k in 2017, they’re now going for $5.5k.

Last time I managed to mix and match valve locks from another salvaged part off eBay. But I have no idea how much mileage that block had. These seem to peter out after ~150k miles and a handful of track days 😔

 

Update:
Found a salvage block from a newer 996TT model year. Noticed that the block has a superseded part number *and* removes the little hole in some brass fitting where the old one leaked. Seems like Bosch may have known it was a failure point?

Old part: 265 255 027 / 996.355.755.47
New part: 0 265 255 101 / 996.355.755.63




I can't seem to locate on the 5.7 ABS diagram what this little brass fitting would be. It's not the two spring pressure reservoirs; those are at the bottom. 🤷‍♂️

 

TL;DR - hot-wired the pre-charge pump motor with wires from the battery to perform a thorough bleed after replacing hydraulic unit, and spongy pedal went away! Car brakes great now.

After replacing the above busted ABS hydraulic unit with the newer one, my normal bleeding procedure did not work. Usually I do a Motive pressure bleed or a brake pedal bleed with speed bleeder valve nipples on the calipers... but neither of these worked this time since this job opened up a lot of the hydraulic circuit. Pedal was still spongy with obvious air in the circuit. My suspicion is pedal and Motive pressure bleeding methods didn't move fluid through the system at high-enough velocity to get bubbles out from far upstream, near the hydraulic unit where the hard lines are at their highest vertical position in the car.

I did a ton of research on my Durametric tool and reached the same conclusion - the bleed procedure in the software doesn't actually do much. It causes some valves to click in the block but otherwise doesn't actuate the charge pump.

So I studied the above 5.7 ABS diagram and inspected all the lines in the actual unit. Determined the charge pump is part of the same section of the circuit as the brake pedal and master cylinder, meaning: you shouldn't need to actuate any valves to have the ABS pump's pressure arrive at each caliper. If it did require actuating valves, brakes would not be a fail-safe system.

So with the help of a friend, I decided to hot-wire the pump with 12V from the battery directly using wires. The pump's connector is easily detached to give you access to the prongs with normal ~12 gauge copper wires that you can run from the battery nearby. I checked the polarity of the pump in the wiring diagram and confirmed red/black = positive 12V, brown/black = ground. I attached the Motive pressure bleeder to the reservoir and pumped to ~10psi as recommended by the Porsche service manual. I opened the outside nipple at each caliper and then shorted the cables to the charge pump for about 5-10s. Fluid went through very quickly, and with it brought out the bubbles that I suspect contributed to the spongy pedal. Now car brakes like new

Wiring diagram for reference:

 

Bump. Undergoing the same surgery. Def need the 6 line variety of hydraulic block but also it needs the pressure switch port/switch on the front side. There are 6 line blocks without that you do not want.

 

Good luck! I was able to find the donor blocks on eBay. I actually have an extra one I’ve never used if you’d like, but it has the hole that leaked in my first two.

 

If its available sure, hoping to refine the information a bit more since it seems folks over on RL are having this pop up often now too. Mine was Porsche part number 996.355.755.58, which had Bosch hydraulic block 265.225.101 on it. It is leaking from the same spot that should be plugged according to your photos earlier in this thread, but looks like it still has the potential to leak.