carvalag

Last weekend I was supposed to enjoy three fun days on the track. I got my CPO 06 997 C4 coupe with manual transmission prepared by installing yellow pagid brake pads and flushing the brake system (I used ATE TYP200). The brake pedal felt mushy after that. I bled the brakes again, the pedal got firmer, but I started to notice that the brake pedal would sink to the floor if I held it pressed for a few seconds. Suspecting of a problem with the master cylinder (MC), I brought my car to the dealer for an inspection and possible replacement of the MC under warranty. The dealer flushed the brake system one more time, this time with ATE Super Blue (which as I understand is the same thing as the TYP200 but blue instead of gold) and bled the ABS. This time around the pedal felt much firmer and wouldn't sink, so I drove 5 hours all the way up to the track site. In the last toll booth on my way, though, I noticed that the sinking pedal came back. By then, the nearest dealer was 100 mi away, and no dealer in a 300 mi radius had an MC in stock anyway so I decided to swallow my disappointment and drive back home, leaving my plans of track fun behind me. I got my car back to the dealer and they're currently replacing the MC. I'll let you know later whether this will solve the problem or not. I'm not particularly optimistic, though, because the dealer gave me a brand new (1,300 mi) loaner Cayenne to use while they fix my car and, guess what, the Cayenne's brake pedal sinks exactly like my car's! I did a search on web forums and found other people who noticed the same thing, including in brand new cars still sitting at the dealer's parking lot (check https://rennlist.com/forums/997-turb...th-brakes.html).

I think a sinking brake pedal is tricky to notice because the pedal feels firm at first, and under normal driving the car brakes just fine. You'll only notice a problem if you deliberately keep pressing the pedal after you come to a stop, which I only did after I first flushed the system and was testing for potential problems. I think this problem may go unnoticed for months if you're not paying special attention to your brakes.

Do you test your brakes regularly for a sinking pedal? Is this a cronic problem with some of the newer models, or just "normal"? If "normal", how low can you (or the brake pedal...) go???

MJones

Not a normal condition

Best guess...Master Cyl

mobonic

Im having this same issue now, and im worried.

Please let me know if it was the MC.

I hate to spend the money to replace and it not be the problem.

I have already wasted 4 liters of SRF trying to diagnose the problem.

Thanks!

rosenbergendo

Had this on my Cayman S, pre GT3. The problem was never properly fixed even with SRF and new pads.

p-cardriver

You might need to bleed either the ABS (Or maybe it just has to be activated during bleeding, I can't recall) or the clutch (same fluid system).

We had this problem the first time the dealer bled our brakes before a track event. Took it to a real mechanic, he bled it properly and he's maintained and teched it since.

BostonDuce

When you bled the brakes did you depress the brake pedal all the way to the floor with the bleed screw open?

BD

carvalag

Wow! You guys resuscitated a discussion that I posted almost an year ago. It was indeed the master cylinder, as MJones guessed. I had it replaced and the problem went away. Well, sort of... I have to say that some level of pedal sinking does occur even in brand new cars. At the dealer I've tried it in brand new 997, Cayman and Cayenne. Their brake pedals all "sank" somewhat if I kept them pressed for a while with sufficient force. I discussed the issue with one of the Porsche technicians at the dealer, who confirmed that up to a degree this is normal in cars equipped with ABS. I've also tried it in my 08 BMW X5 with the same sinking pedal result. So, as I would love to give you guys a "black or white" answer, the matter of the fact is that the assessment of whether the brakes are working properly or not is subjective. The brakes just don't feel right. Mine was going almost straight to the floor after a couple seconds and with not too much force. After I had the MC replaced, the pedal would still sink, but would not go to the floor and would require much more force. I hope this helps.

cviles

I had a similar problem on my Boxster -- except the first sink to the floor happened coming to turn 10a at Road Atlanta. I'm surprised I kept the car on the track, actually.

Anyway, the problem was a disintegration of some parts/seals inside the master cylinder. Porsche replaced it under warranty. However, that didn't solve the problem completely -- bits of the material shed by the cylinder contaminated the ABS controller which eventually had to be replaced as well. Eventually Porsche replaced that as well as a 2nd MC but told me they wouldn't be providing any more warranty coverage on the vehicle for this problem in the future.

My pedal doesn't sink to the floor anymore and braking feel is nice and firm they way it should be.

My 997 has never had the sinking pedal problem.

mdrums

Definitly the master cylinder.

Also stop using ATE fluid, especially the blue junk...get it out ASAP.

When they replace the MC flushing the system really good and use Motul RBF600 or 660.

mobonic

I have already flushed the blue out with 5 liters of SRF... wasted

Well the best guess we think are:

1. Bad master cylinder
2. Bad brake booster
3. Fried brake caliper seals