coryf

How worn is the differential? With the stock diff plates that wear out on track quickly, the inside rear wheel wants to spin exiting the slower corners. The TC even when turned off will use the rear brake to grab the inside rear rotor to slow the wheel spin and over heat the brakes. A lot of gt3's rear calipers are much darker than the front because of this overheating. Rebuilding the diff will help a lot. The rear gt2/gt3 cooling ducts are helpful as well. I have seen this quite a bit at one of our local tracks with a lot of braking zones and slow turns. Might not be the issue but worth a look. Jack up on side of the car and spin the rear wheel to see how much break away torque the diff has.

fusi23

It happens the same in my 07 GT3.
I tried different type of discs, pads, hoses and still.
Never rebuid the calipers.

It will be the next thing I'll do.

audipwr1

Does your shop have different set of calipers around?

In these situations you have to start eliminating variables

Jake951

I have an OS Giken LSD that's two years old and works fine as far as I can tell. My rear calipers are the same color as the front and show no signs of overheating, although I have seen this discoloration on other cars. In fact I've checked caliper temperatures with temperature strips, temperature sensitive paint, and an IR pyrometer. The temperatures never go much over 300F.

Unfortunately no.

KA MOTORSPORT

To me it sounds like your calipers need to be rebuild properly using the correct parts and technology.

We rebuild calipers here so we could do that for you.

coryf

Have you checked the break away torque? It may have been mentioned before, but are you mechanically bleeding the brakes or using some sort of power bleeder?

Jake951

I haven't checked the break away torque but I'm certain that the diff is not the problem. There are absolutely no signs of excessive heat in the rear calipers, as confirmed by temperature sensitive strips and paint on the calipers, and IR pyrometer measurements.

Bleeding is always done mechanically with somebody in the car working the brake pedal.

Protocav

Could this just be a massive knockback issue from worn out wheel bearings? The tap of the brake before the turn that you suggested would validate this condition.

Jake951

It's definitely not wheel bearings, although I did investigate this possibility earlier. Front wheel bearings were replaced a while back, although not the rear. The thing is that the "long" brake pedal persists even when the car is standing still, as long as the brakes are hot. As I said before, you can see a visible gap open up between the pistons and the pads when you let off the brake, and the tap closes it up momentarily.

Igooz

1) for me back to the basics: are you sure that your system is bled properly? There are always theories about bleed process, but some believe in modulating the ABS valves while pulling vacuum to get all air / all fluid out.
2) Wilwood makes inline residual pressure valves. You can get them in 2/5/10 psi. I have used them in race cars but cars built from scratch and not factory cars such as yours. The primary purpose has to do with brake fluid drainback.

Jason @ Paragon Products

I wonder if this could be some sort of venting issue causing a vacuum. Maybe get an extra reservoir cap and drill a tiny hole in it and see what happens.

Jake951

The ABS valves are always actuated as part of the bleed process.

I don't think it involves hydraulics at all. When the calipers are completely removed from the car with no brake lines attached, you can still see the pistons pull back after they are pushed in. It must be the seals that are pulling them back in.

mm450exc

I will give this a try on the next track day!

http://forums.pelicanparts.com/porsc...-question.html

mm450exc

Problem found! Very much uneven pad wear. Pad 10mm at top and 5mm at bottom! So was basically running on 50% of the pad - heat!

KA MOTORSPORT

Ideally the normal amount of longitudinal taper should only be 1.5 mm. Extending the pistons would definitely create travel (not sponginess) in the pedal. But this is a known issue on the Porsche red calipers and flipping the pads regularly is a bandaid.

When the pads wear down past 60%, more heat gets transferred to the calipers as well. The compound also serves as some insulation for the caliper as well. Very much a hydraulic issue as well as the fluid and piston seals get taxed.

A lower wearing pad which generates less heat would be good to consider next time around.