Brake Fade
 

My son had my car at the track yesterday and he indicated that he was having brake issues. He said that the brakes would be very consistent, then during the a random braking zone the brake pedal would go to the floor, and then the next braking zone the pedal would return to normal.

I bled the brakes about 2 events back with Motul and did not have any issues during those events. I have a slight drip where the reservoir ties into the master cylinder, checked all the lines and they were dry.

Anyone know what would cause this? I don't want to be caught in a high speed environment and have no brakes.

Wheel bearing. Most likely rear.

When the pedal was soft, was it either after a specific turn, or after he drove over the curbing?

Random, it made him nervous so he tested it on a couple of straight aways and had an issue once. Did a check on the bearings and bushings last month and they were all in good order, no movement in the bearings, rear bushings did not show signs of deterioration and the fronts are spherical.

Either the fluid is becoming compressible (air, boiling), there's a leak in the system (you don't think so), or the hydraulics are malfunctioning (fluid getting around the piston in the master cylinder). If the master is old, just replace it as it's pretty cheap and aren't "forever parts". Was a brake dragging or anything that could cause extra heat to get into the fluid?

Pads look good I don't think there was a drag issue but I will pop them out and check the claipers to see if there is any sticikiness. My original thought was the master cylinder but I wanted to get other input to confirm. It is one of a dwindling number of original parts on the car.

what fluid are you using and what pads?

Does the pedal get firm after the first application to the floor? If so, Van is headed in the right direction or you need a new master cyl.

Brake fade due to boiling fluid normally doesn't cure itself between brake zones.

I would guess that it's pad kick-back. Usually when the fluid boils, it boils, and won't come back until they cool down. But pad kick back will be totally intermittent.

Maybe he was hitting curbs hard? My car, running over the curbs at Road America, tends to push the pads in. I got in the habit of doing gentle quick taps after curbs down the next straight to make sure I had pedal.

Did this happen multiple times during the same session on track? If so, was it in the same braking zone across multiple laps? What brake pads?

When you say the pedal went to the floor, do you mean that it traveled farther than normal, or that it just traveled with little resistance on the pedal? You can generally feel the difference between compressing a sealed system with an overheated pad, compressing a system with boiled fluid, and compressing a system that is losing fluid. I've heard drivers describe the pedal in each case as going to the floor.

Pads are Pagid Orange

Fluid is Motul 600

It maybe pad kick back, but to be on the safe side I am going to replace the master cylinder.

Pedal to the floor means no pedal pressure what so ever.

Tried pumping some times returned a couple of times no.

Brakes are 996TT calipers up front, standard 951s in back.

Bob, does your car have ABS?

Silly rabbit ABS is for kids :D

Not trying to trying to start a discussion of the merits or lack thereof of ABS on a track car but IF you have ABS and you happen to break a front hub so that the part of the hub with the ABS ring doesn't spin any more or very little, it will feel like you're losing your brakes. It may spin at times and the brakes will work and other times it won't spin hardly at all. Needless to say it freaks out the ABS. Happened to me on the 968 Turbo RS at the Rennsport Reunion at Daytona.

Just my way of saying no ABS, car came with it removed. At times I wished I had it as thresh hold braking can get iffy when the brakes start heating up. I had the broken hub happen to me on an earlier car and had forgotten about it. Thanks for the reminder.