ScootCherHienie

when i press on the brake pedal before starting the engine, there is a bit of progressively increasing stiffness until the pedal reaches the limit of travel... maybe 50% of the way to the floor. Once at that point, you can put all the force you can muster and the pedal will not sink further. if you start the car with, say, 50% of full pressure you would ever use, as soon as the engine begins running, the brake pedal slowly sinks towards the floor (vehicle sitting still with foot pressing brake pedal). In the distant past, that would mean a master cylinder rebuild. these days, braking systems are called on to do so many more things like ABS, stability control, handling assistance, save-your-*** tricks, etc. I suspect that this might even be normal for some reason. While driving, you don't notice it unless you intentionally press harder than normal on the brake pedal while stopped. if this is normal, why?

deilenberger

Sorta normal. It's the brake booster at work. With the engine off - there is only residual vacuum in the booster - so you typically get one boosted press - then it's unboosted. Once the engine is started you have power-assist which gives a bit of a non-linear feel to the brakes - and if there is any air in the system or the brake hoses are getting old (they swell up under pressure) - it compresses due to the greater force applied by the booster.

When is the last time your brake system was flushed? Porsche calls for it every 2 years (as do almost all German vehicles.)

ScootCherHienie

All fluids except power steering were changed in June 2017 (brakes, differentials, transmission, transfer case, coolant), so 2 years will be this coming summer. 100,000 miles on the S, and it is driven 5000-6000 miles a year.

Brake hoses have NOT been changed (2008 model year)... will the 4 short rubber hoses at the calipers stretch so much in this much time/mileage that the brake pedal could sink from mid-travel to the floor with pressure on the pedal? I DID feel the brake hoses streching (and could visibly see it too) on my 2001 Harley when it was only a few years old and changing to steel braided brake hoses stopped all of that feeling.

deilenberger

I'd first do the fluid flush - if it still does it - then I'd be under the car while someone presses the pedal with the engine running - looking at the brake hoses. If you see one move appreciably chances are it's ballooning.

BTW - my '11 CTT did this once - in my driveway. Since I got the fluid flushed at the dealer (who can activate the ABS system - and in the CTT the emergency boost, adaptive cruise braking) - it has never happened again.

Petza914

If the pedal sinks while the engine is off, it's the brake booster or a line to the brake booster. Should be very hard with the engine off. With the engine running, if it feels mushy or sinks, it's usually air in the system, but if this is a new sensation that hasn't been happening since the fluid was changed back in 2017, and just started, but the braking system hasn't been opened up since, it could be a failing master cylinder or bad lines, or a leaking caliper piston. Look carefully inside your wheels for any drips of fluid (leaking piston), also check all the brake line connections between the metal lines and rubber lines and calipers, especially the front ones on the inside of the wheel carrier about half way down as I've seen these loosen their own.

You can do a layman's bleed of the abs system simply by engaging it a few times. Find some loose dirt or gravel, come in hot onto it and lock them up so you slide and the abs engages. Do a few passes, then rebleed the brakes.

If none of these change anything, do as suggested and inspect the rubber brake lines while someone else pushes the pedal. Usually rubber lines fail by becoming brittle and cracking (leaking) which will create a sinking pedal, or by internally swellibg and limiting fluid flow to and from the caliper, which either makes the brakes less effective since the fluid pressure doesn't push the caliper pistons efficiently, or makes them hang up and drag as the fluid doesn't readily release the pressure on the pistons. Rubber Porsche brake lines are dual rubber layers with a reinforcing fabric layer sandwiched between, which are better than normal rubber brake lines but still jot as good as stainless. Stainless lines are Teflon inside, which doesn't swell over time, and the braided stainless material gives better pedal feel. Make sure the set you buy is DOT approved and has a coating over the braided stainless so it doesn't rub and abrade anything it touches or the line itself.

I install braided stainless lines on all my cars (do it once and never worry about it again) and for the PCCB brake upgrade on the Cayenne TTS, I used the set from ECS Tuning which looks to be of excellent quality and fit well.


Front stainless line prior to wrapping protective coil.




Front with coil




Rear from outside with coil




Rear as seen from inside




Rear bleeding

ScootCherHienie

Thanks for the info. I didn't notice this for about 18 months after we bought the 08 957. And then, I only noticed it because my brother's X5 V8 was doing this when I drove it and I noticed it there first. It is about the same age as the 957 (2008) with similar miles (100k). So the next time I got into the Cayenne, I did the same thing I did in the X5 and the 957 did the same thing.

Decades ago, before ABS, stability control, traction control, and all the other things they use the brakes for didn't exist and there was only a dual master cylinder, brake pedal, proportioning valve, and maybe a vacuum booster; if there was air in the brake lines, you could pump the brake pedal maybe 3-4 times and it would finally firm up pretty well, even if the pedal went to the floor on the first stroke of the brake pedal. I'm guessing all the fluid control from the system that handles ABS-etc. may have changed the feeling of air in the system at the brake pedal compared to brake systems without all the extra control?

In my case, the pedal never goes all the way to the floor, and pumping the brake pedal like you would in the old days if there was air in the brake lines has no effect on the "stiffness" of the brake pedal. I did test the ABS system on a dirt and gravel surface after the rear brake pads were replaced last summer and got maybe half a dozen good actuations of the system in a straight line and while circling left and right so that only the 2 unloaded wheels get ABS actuation. That worked as expected.

The pedal is hard as a rock with the engine not running and no amount of foot pressure on the brake pedal will cause the pedal to sink towards the floor with the engine off. There's no brake fluid around the brake lines or calipers and no evidence of seepage anywhere including the rubber brake lines. Rear brake pads were replaced last summer and rotors were surfaced (I hate changing rotors if the ones I'm using don't warp and they are thick enough to resurface). The shop that did the work last summer said that when they compressed the caliper pistons on the rear brakes to install the new pads, the brake fluid came back up to the proper level indicating no leaks in the brake lines or calipers. But I didn't notice the brake pedal thing till later, maybe last September when I was driving my brother's X5. I had never noticed this on the Cayenne before noticing it on the X5.

If I start the engine with extra pressure on the brake pedal, you can feel the pedal "soften" as soon as the engine is running and the pedal will slowly sink with constant pressure on it--not all the way to the floor, but maybe 85% to 90% of full travel (guessing about that as the pedal has never touched the end of travel so far--at least I don't think so). But if you use only enough pressure to hold the car still with the engine running and in "drive" (transmission), the pedal doesn't sink towards the floor. The pedal only "sinks" when you add more pressure than necessary to the brake pedal.

Petza914

Your excellent and thorough description is exactly how my 957 TTS behaves so I think you're fine.

G03RT2

Instead of starting a new thread I thought I would reply to this one.
I have a new-to-me 2008 CTT, and I'm currently experiencing exactly what the OP says he is experiencing.
After a few pushes when the car is off, the brake pedal is hard as a rock, but once the car is started, and vacuum is being made, you can push the brake pedal and not only does it feel easy or almost soft, after you get to the point of it stopping it's almost like the brake pedal is being pulled away from my foot for another half inch. If I put pressure on it after that I can continue to depress it almost all the way down. Haven't had to do any emergency stops with the car yet, but it's not very confidence inspiring.

The question is... Is this normal?

I power bled the system twice. Changed the master cylinder (not my favorite job to do on this car) and power bled the system another 4 times after that. Even put 25psi in the motive pump, tapped on all calipers and brake lines and bled one last time. With the exact same pedal feeling as before.
Also, if anyone is curious, PIWIS 3 does not have any way to actuate the ABS pump to try and shake air out of it.

Petza914

Mine does this to some degree also, but don't think it ever sinks all the way to the floor, so I don't think that's completely normal. Do the brakes seem to work fine otherwise until you're stopped and then the pedal sinks?

Maybe find a place where they're building house and the street has a bunch of dirt on it from the construction vehicles, and do a few panic stops where you can feel the ABS kicking in. This will let you engage the ABS with less vehicle speed than on a normal street or wait until it rains and do it in an empty parking lot. This will engage the ABS pump and move any air through the system. Then bleed the calipers one more time.

G03RT2

I appreciate it the response. I got pretty deep into this issue. Listened for brake booster leaks and all of the above. I think at this point I'm going to accept it as "normal" because I can slam on the brakes and it stops quickly like it should. Not going to call it resolved just yet, but I'm going to drive it for a while and see if I just get used to it.