Advice needed: weird vibration + long brake pedal travel only when hot on track...
 

I was hoping to consult the Rennlist brain trust to get some advice on tracking down a braking issue I'm having on track with a 991.2 GT3. The sequence of events are as follows:

1) Brakes feel great and are smooth when cold and I start out the session. I progressively get them hotter over the course of a couple laps, and they still feeling smooth / great.

3) Once I get them really hot (pushing it hard for a couple laps), I start to get some slight pedal vibration / pulsation feel under braking.

4) As I keep pushing it and drive them hotter still, then the vibration gets noticeable all the time, even when not touching the brakes! And here's the kicker: once I hit this step (4) where the vibration becomes noticeable all the time, the brake pedal travel starts to ~double but only if I don't touch the pedal for a while (e.g., after a long straightaway). (!!) The feeling is extended travel, but then the pedal becomes firm at the end of that travel once I take up the pedal distance in that single stroke.

I've ruled out the following (but open to putting them back on the table):
- Wheel balancing (because there is no vibration at high speeds when everything is normal temperatures)
- Alignment (because there is no vibration at high speeds when everything is normal temperatures)
- Pad deposits (because the rotors look clean / smooth, and when the vibration issue gets worse I don't actually have to touch the brakes for it to happen, and replacing the rotors didn't change the problem)
- Warped rotors (because the rotor swap didn't change the problem)

My current working theory is:
- As the brakes / wheels / hubs get hot, something is developing some runout (wobble about the rotating axis), allowing the rotors to move slightly relative to the calipers. This wobble would explain the vibration.
- The wobble of the rotor relative to the caliper would then push the pistons of the calipers in slightly (pad knockback), which would explain the longer pedal travel after periods of not hitting the brakes.

But what I can't pinpoint is what component actually has the problem... I originally thought it was a problem with the brake rotors warping when they got hot, but I proactively got all 4 rotors replaced hoping it would resolve the issue, but the problem remains.

The only thing left I can think of that can cause rotor wobble relative to caliper is a wheel bearing problem: perhaps once a bad wheel bearing gets hot enough, it develops enough play to allow some runout, creating the vibration as well as the pad knockback (since the rotor and wheel are on one side of the bearing, and the upright and caliper are on the other).

But has anyone seen a wheel bearing issue on these cars that only shows up at high temperatures? The dealer is going to have a hell of a time tracking down such an issue, for obvious reasons...

Thanks for reading, and thanks in advance for any help!

Maybe bleed the brake lines?

Faulty brake booster or vacuum leak?

Mech, are you running OEM or aftermarket brake set up?

Thanks. I forgot to mention in my write-up that I re-bled the brakes after the first track day where this started happening just in case it was air related. Same issue at the next track day... Air in the lines could explain a soft pedal, but I can't see how it explains the vibration that goes along with it.

Thanks for the suggestion! Any thoughts on how that could manifest itself this way? I'm not that familiar with how the booster system works.

Aftermarket. So if I can't figure it out, next step will be to take it back to stock and see if the problem goes away then... hoping to avoid that if possible, but I don't have much else to try.

Thanks. The trouble is the wheels and bearings don't show any play when the car is cold... I'd have to come off track HOT, then quickly jack the car up, and I'd maybe get to test one or two wheels before it cooled down. I tried that the last track day and maybe got one wheel checked before it cooled and didn't find anything. But later that day I isolated it to the drivers side of the car from the turning and braking experiment and didn't get a chance to do a hot pit check again before running out of time.

sending you a PM now

Brake fade does enhance the feeling of the brake pedal sinking because you press on it harder. The fact that after repeated heating cycle, you feel a vibration even without braking might be caused by a faulty tire. The tire might get out of round after heating up. Next time it happens, pull into the pit area, jack one corner up high enough that the wheel barely off the floor, then rotate it. If one area rubs on the floor, then you have a high spot. Do it on all 4 wheels to get an idea which one has the most our of round problem.

Boiled fluid, stuck caliper
Puck issue https://rennlist.com/forums/991-gt3-...-ceramics.html

Just some other thoughts: have you checked the caliper pucks? Also, check that the pad spring retainers are lined up just right. Also, what pad compound are you running, and are you sure it has a high enough temp rating?

Thanks everyone for the advice, much appreciated. I plan to do a tear down this weekend to look over everything carefully. FYI I'm running Castrol SRF fluid.

Hello,
In reference only to your brake fade/soft pedal issue. As others have stated once the brake fluid reaches its boiling point, the liquid turns into a gas/vapor, gas/vapor is compressible, a liquid is not. That is why you're potentially getting a soft pedal. This problem will persist since the gas/vapor will remain in the lines until you bleed the braking system. You can further avoid this by stepping up to our RBF 660 which has a higher boiling point than the SRF.

Below is an excerpt from conversations we've had here on Rennlist in the past about Motul RBF products vs the SRF that you may find useful in track applications:

If you flush your brake system often, RBF 600 or RBF 660 will out perform the SRF:

RBF 600: Dry Boiling Point: 594 F
RBF 660: Dry Boiling Point: 617 F
Castrol SRF: Dry Boiling Point: 590 F

If you're looking for an extended drain product with a longer service interval versus higher short term performance, the SRF is a better choice as it has better moisture stability when water is present in the brake system. Brake fluid is very hygroscopic which is prone to absorbing water. Water lowers the performance level of brake fluid when present in the system.

Our position is in most racing applications, water is a non factor since brake fluid is drained regularly so we put more stock in dry brake fluid performance vs wet. And as you can see, the RBF 660 has a 27 F advantage vs the SRF.

All wet boiling point tests measure brake fluid at 3% water concentration- this would be considered a long drain service specification and again, not really a factor in short drain race applications.

If you have any questions, feel free to drop a line here.-Nick