Brake pedal stops normally then slowly fades to floor. Master cylinder? Booster?

[lskolnick]

👍👍👍👍👍👍👍👍👍👍👍👍🙌

[chriscpm]

Originally Posted by Rotmilky

Well, I replaced the ABS with a used one from a wrecked 2001 base. It had the same part number as mine (it's a 2001 S), so I figured it'd work. Installed it this morning, bled the brakes (and bled and bled and bled...there's a lot of air in the ABS to get out). Took her for a test drive and the brake pedal is back to normal. Woohoo! Nice to have normal brakes again.

Lucky thing there are a lot of used ABS units out there at a reasonable price (I paid $70). A new one from Porsche is about $3000.

So, if anyone happens across this thread in the future and has a similar issue. If a replacement master cylinder doesn't fix the brake fade, take a look at the ABS.

I know this is an old post. But I'm going down this same path. A new master cylinder didn't fix it I'm going to do the ABS pump next. Another symptom is the rear pads wore faster than the fronts after some track use.

[jcslocum]

That's a long, frustrating but good result!!!

[Gilles]

Congratulations for finding the root cause, and thank you for posting your results!
.

[aussieboxy]

I had pretty much the same symptoms & it was the booster.

[michaelpetersen3]

Rotmilky, thanks for your detailed outline of the progression! I also have an 01 986S that is exhibiting the same problem. It happened during a recent track day, where we were working the vehicle (and ABS) to its limits most of the day. I'm running race compound slicks with stock pads so the forces (and pressures) in the braking system were at a maximum. Based on your journey, I'm actually going to start with a used ABS pump and see if that corrects the problem. I'll make a point of reporting back my results and hope somebody else can benefit from the insights.

[michaelpetersen3]

Just wanted to close out on my issue above. Thanks to eBay and $75 I replaced the ABS unit with an identical part number, i.e. had the same stamped part number and was from the same model year vehicle ('01 986 S).

After replacing the ABS unit and a brake fluid flush to all four corners, the brakes feel like new. No sponginess and they do not slowly compress after the initial contact.

When it failed, I was running stock pads but with race class slick tires (Toyo Proxes RR), and intentionally pushing the brakes to threshold several times on every lap (including occasional ABS activation). I suspect I was pushing the ABS to its limits. It would have had max pressure and coefficient of frictions of the tires was beyond what the engineers tested for 20 years ago.

Hope this helps the next guy...

(BTW, as most of you know, the brakes on the 986 and 996 are virtually identical other than tuning)