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A few months back I posted this thread, "Why does my brake pedal go soft on the track?" (https://rennlist.com/forums/997-gt2-...highlight=soft). I now know the answer but I'm not sure how to fix it. I'm hoping somebody in this forum can help.
To recap the original problem, the brake pedal would start to get "long" on the track. As the brakes got hot, it took more pedal travel to get braking as a track session progressed. A quick tap on the brakes before hard braking would temporarily restore the brakes to normal, so that became my braking routine on the track. On the street, the brakes never had the problem and letting the car sit overnight after a track day would restore the pedal back to normal.
After eliminating a lot of potential causes, including brake fluid, master cylinder, rotors, pads, brake lines, and everything having to do with hydraulics, my shop finally discovered that the cause is the brake piston seals pulling back the brake pistons so that a gap opens up between the pistons and the brake pads. It is not much, a fraction of a millimeter, but it is clearly visible by eye. A quick tap on the brake pedal closes up the gap for a moment before the seals pull the pistons back. We were able to replicate the problem on the bench with the brake calipers completely removed from the car. The piston seals were Brembo seals and the calipers are the OEM GT3 calipers.
My shop rebuilt the calipers with new StopTech seals instead of Brembo. Guess what? The problem still remained and was even worse than before. It was really bad now even on the street. A suggestion was made that the seals, being rubber, have mechanical "memory" which causes them to pull the pistons back into the calipers when the brake pedal is released. A trick that was suggested to fix it was to let the car sit for an extended period of time with a broomstick pushing down on the brake pedal to force the seals to form new "memory". This is what I did:
After 24 hours I took the car out on the street and the brake pedal was now dramatically better. The pedal was rock hard and better than ever. This seemed to confirm the theory about the seals.
Unfortunately the great street behavior with the new seals still did not carry over to the track. The brake pedal would go "long", just like before, and would recover overnight. I still had to do the quick brake tap routine on the track. I've tried the broomstick some more, for days at a time, but still have not improved the track behavior by much.
Now that I know it is the seals, I am at a loss as to what else I can do. My shop has rebuilt lots of calipers with new seals, including other track GT3's, and my car is the only one with this problem. I know of at least one other GT3 that has the same problem, so it is not an unknown issue.
I have now run out of ideas. Do any of you experts have any more insight into this problem and how to fix it?
Remove the outer seals. Cup cars don't run them. Interesting I seem to rcall the previous thread suggesting knock back bit this is the seal vs floating rotor causing. P
I've rebuilt a few calipers and a not and this issue. Makes me wonder of the inner seals are worn and getting pulled in too easy. The action on the outer seals is not strong.
Did they rebuild the inner and outer seals and inspect the Pistons and bores?
Soft or spongy pedal is always going to be some kind of hydraulic issue and caliper seals play a big part in the consistency and feel of brakes. This is different from knockback.
Seals need to be serviced every 5000 km or if there is any sign of leakage (that’s what we do on the cup car & race calipers). Have to use proper seal conditioning fluid and soak it in overnight too, otherwise sometimes the seals don’t work right. Street GT3 and 997 Cup have standard calipers with a standard piston seal and dust boot….burning the dust boot won’t do much harm but if the seals are worn, then performance will go down.
If there is any damage to the pistons, we replace them immediately.
From my experience, Brembo seals work best in Brembo calipers even though sometimes a pain to source. Other brand seals in most cases, makes it worse.
Otherwise, the only other time I have seen such symptoms is if master cylinders need service. Happens on the Cup cars but not so sure about the street GT3.
All calipers, but not every piston on each caliper is doing it.
What pads are you using; Pagid(?) and what fluid in addition?
I've tried PFC08 and Hawk DT70 pads, and the results are the same. I've also tried Motul RBF660 and StopTech RTR660 as well as lower temperature fluids, all with no improvement.
Did they rebuild the inner and outer seals and inspect the Pistons and bores?
Yes, all inner and outer seals were replaced and all new pistons were installed in the most recent rebuild. The ceramic heads on the old pistons were showing cracks and the old pistons also had some scoring on the sides.
I wonder if my brake calipers are bad. Is that possible?
All calipers, but not every piston on each caliper is doing it.I've tried PFC08 and Hawk DT70 pads, and the results are the same. I've also tried Motul RBF660 and StopTech RTR660 as well as lower temperature fluids, all with no improvement.Yes, all inner and outer seals were replaced and all new pistons were installed in the most recent rebuild. The ceramic heads on the old pistons were showing cracks and the old pistons also had some scoring on the sides.
I wonder if my brake calipers are bad. Is that possible?
A proper brake fluid like Castrol SRF or Brembo HTC64T will definitely help.
Who performed the servicing of the calipers and how much track mileage do you have on the calipers?
The Porsche OE red calipers have an inner piston seal and a dust boot. The dust boot and ceramic puck only protects the inner seal but has no real effect on pedal feel. If the pistons were scored, then the calipers should be inspected too for damaged bores.
Soft or spongy pedal is always going to be some kind of hydraulic issue and caliper seals play a big part in the consistency and feel of brakes. This is different from knockback.
Seals need to be serviced every 5000 km or if there is any sign of leakage (that’s what we do on the cup car & race calipers). Have to use proper seal conditioning fluid and soak it in overnight too, otherwise sometimes the seals don’t work right. Street GT3 and 997 Cup have standard calipers with a standard piston seal and dust boot….burning the dust boot won’t do much harm but if the seals are worn, then performance will go down.
If there is any damage to the pistons, we replace them immediately.
From my experience, Brembo seals work best in Brembo calipers even though sometimes a pain to source. Other brand seals in most cases, makes it worse.
Otherwise, the only other time I have seen such symptoms is if master cylinders need service. Happens on the Cup cars but not so sure about the street GT3.
We only have the problem when hot on track. When cold the brakes are rock solid! You reckon it's the seals? Replace with standard brembo seals or does brembo make anything better?
08-18-2015, 12:32 PM
Why my brake pedal goes soft on the track
