Oops. Missed that one. I does not seem noticeable different if I pump the brakes after it bottoms out. Pretty much the same slow bottoming out no matter what I do.

well. in most cars, when you have your foot on the brake and you turn the engine off (which kills vacuum) the pedal will slowly drop to the floor. makes me think vacuum issue. or, perhaps an issue w the abs pump? they usually throw a code, however.

The factory "rubber" brake lines have steel braid on the inside under the rubber.

Then I agree with you 100% that it must be a leak where the fluid goes from one part from the system to another. My vote is then that the problem lies in the ABS or PSS (if you have it) systems. If you have PSS my vote is there since the car can activate the brakes without you touching the brake pedal.

I don't have PSS or TC. Just the ABS. In the ABS, there are 3 parts I'm considering. (1) There's the proportioning valve on the side of the ABS, (2) there's the main ABS itself (with the ~5 brake lines flowing in & out). That thing is a nebulous black box...a new Porsche parts costs about $3K, but a used one can be found in EBay for maybe $30. (3) Then, there is the piece along the sidewall. Porsche calls that the intermediate piece. I'm not sure what it does. The vacuum comes through this piece and there are 3 different steel lines connecting to it.

I'm guessing one of the three pieces I listed there must be the culprit by process of elimination.

Since I have two boxsters and one of them has a perfectly normal brake, I'm considering swapping pieces off of the working boxster until I can eliminate the problem. My worry, though, is that I end up with two Boxsters that have brake problems.

watch part numbers - many variations of abs pump.

In my ongoing saga of the brake, a new proportional valve did not fix the problem. Err, actually it was a used valve. There weren't any in the US and I didn't want to wait for the part to come from Germany. I guess if the failure rate on a part is so low that Porsche doesn't keep them in country, it's doubtful mine would need replacement.

Looks like I'll put in a different ABS to see if that fixes the problem.

As I've thought about this more, I noticed a curious thing about the bleeding tonight. I got almost no air out of the system. I saw one or two tiny bubbles but nothing larger than a sand grain sized bubble. Since I did open the system up to replace the valve, I figured there'd be more air in the line. It makes me think that air is getting trapped in the ABS system and not bleeding out. I don't have a durametric to actuate it. In the past few weeks, I take the car for a hard drive and make sure the ABS kicks on. I've assumed that is enough to purge the ABS of air. But now I wonder if there isn't air still stuck in there....

IIRC, with the basic ABS system, its a regular system bleed/flush. Nothing to activate

I'm as stumped as you. I'm a mechanic but I'd be referring the car to a brake specialist at this point.

Air in the system doesn't behave like that, it would be a spongey pedal every time, not a sinking pedal when held lightly.

You must be so frustrated!

I don't have much to offer here and have watched this thread closely.

1) Does the pedal firm up and stop drifting if you increase the pressure a bit (not full stomp) when it begins to drift down??

2) You may have gotten a bad, new master. It happens, maybe change it again?

3) Get a used ABS pump from Woody to swap in and see if that fixes it.

1) No...drift seems to independent of pressure. Light touch or firm touch. Similar fade speed.

2) I thought I might have gotten a bad one (it was an ATE). I ordered another one (Meyerle) and it does the same thing. Three master cylinders. All fade at about the same rate. I'm hope I'm not so unlucky as to have three fail on me (two brand new).

3) I've got a used one coming. That's pretty much the last part I can think of to replace.

If that doesn't work, I think I'm going to start pulling parts of the working Boxster till I fix it.

I agree on the air assessment. I don't think it's that. But after trying just about everything I can think of, I'm grasping at straws. I've even started to wonder if there aren't things like brake termites that eat my seals.

While the problem is annoying, the brakes do work just fine for stopping on an autocross course. I don't even notice it there. So, I still use the car. If I were actually racing the car at high speeds and needing long braking distances...that would be another story.

I just resolved this exact problem today. Master cylinder replaced twice before determining it was a bad ABS pump. $50 on eBay and some labour, and she's all fixed up. Make sure you get the correct part number. There are several over the years. Just match your existing pump.

So the ABS pump can be the root of all brake evil. Makes sense that it could be bypassing fluid. How many miles on it??

Less than 100k

Car has 80K on it. I think the pump is original. So...in the grand scheme of car mileage, that's not a ton on the pump. That said, it is almost 20 years old now. I was doing some reading on a 987 forum as well where people were complaining of a similar problem. One guy mentioned that Porsche replaced his ABS and it fixed the problem. Porsche thought that maybe gaskets from a failing MC could find their way into the ABS and cause issues. So, I'm hoping that'll fix it. Really, there aren't any other parts to replace at this point.

I saw some 993 guys with this problem as well. It's not unheard of, but quite rare. I guess we are the lucky few. 🍀 😊

Fixed!
 

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.

👍👍👍👍👍👍👍👍👍👍👍👍🙌