ok, i have spent hours searching the last couple days and looks like my situation is abit different than others i have come across.
on the one day and one trip my cayenne had to fail, it was on a 700 mile round trip to go to my grandmas funeral. was a sobering moment i had to call it 150 miles in knowing me may not make it and had to turn around, missed the whole funeral and was very bummed.

about 100 miles in, my abs failure showed on main screen and then idiot light stayed on after. i kept pushing through assuming it was just a glitch. about 10 miles later, the red "brake proportioning failure" light came on and i lost my speedometer and other gauges, tach still good. i pull off highway as soon as i safely could. first check was the brake fluid level. i was right at the fill line, so hoping for easy fix.
7 miles was my next service station. abs light came on within a couple miles, then the red brake proportioning warning shortly after again. picked up some brake fluid and topped off. fired truck up and still had the abs warning. took another 20 miles or so and here came the brake proportioning warning again. at this point we decided to turn back as we still had 150 miles just to make it back home.
shut off, then restarted to "reset" the red brake proportioning warning a couple times on way back, but once we got to 100 miles away i just used my tach to tell my speed and forged on home.
now, once i get off freeway and to surface street stop and go, the truck was going nuts! seemed to start in 2nd even though i was selecting first, random wheels were applying brakes, and just general nuttyness!

after searching for fixes i have so far-
-pulled back carpets on both sides= dry as a bone!
-checked wheel sensor wiring= no damage
-removed wheel sensors and cleaned= did not change anything. (i did however start truck with sensor disconnected and it said "psm failure" at that point, -so i believe that knocks out a possible wheel sensor failure)
-i replaced battery less than 2 months ago (good gel cell with more cca than stock)
-brakes pads have tons of life left in them
-brake peddle switch working fine

it appears it takes a little while for the "brake proportioning" red warning to come up, but the abs failure (no pcm failure) warning comes up upon start up and the engine cel is on also. i have a generic code reader and it is telling me no codes exist, but it is only capable to read ecu info i assume.

so there it is in a nutshell. luckily it isn't my only vehicle, but i have many projects going on and it sucks to now have a third vehicle down at once! ugh.

thanks in advance for any help. hopefully someone has an answer for me or more things i could check.

 

it is an 04 turbo.

here is a shot of dash to show lights currently illuminated.

 

wow, nobody huh?

well, bought a durametric cable and it came. tried reading codes and for some reason the usb ports are not recognizing the cable. said screw it and finally just unhooked battery for an hour and hooked back up. idiot lights are gone and seems to run fine for a quick trip around the block.
my brake fluid level was a little low when i checked it, but i topped it off during my trip to no avail. i wonder if it was low enough to trip a code, and even after topping off resevoir, it was still trying to force me to get truck serviced?

time will tell. if it comes back and i get the durametric going to find codes, i will let others know the problem.

 

The fact that you lost your speedometer and other gauges when your ABS issues appeared is the smoking gun here.

I would have said to check your battery, but since you recently replaced it, probably not the culprit, but I still would check integrity of battery connections.

As you mentioned, we need to see the codes.

 

I had my first "PSM Failure workshop" display and warning light come up at the beginning of April on my 04 CS. The car ran poorly while the warning light was on. After shutting off and restarting at the end of the journey the warning disappeared. It happened a few times again after that, which I cleared by turning the key off and starting again.
I searched the forums and saw that this may be related to battery failure, but I didn't do anything about it. I saw the warning a few more times, maybe a total of five in the last 2 months.
Last Sunday morning she refused to start and lots of warnings and failures popped up on the dash. I charged the battery for a few hours and got her started. Next morning , battery drained again.
So I have just replaced battery with a new Varta I1.
All sorted !