pearldrum944

So there is this course on my campus that is kind of like a small track (like the size of autocross, I'm pretty sure I'm not supposed to be there) Anyway, After driving around it for maybe 1-2 minutes I hit the brakes before a turn and the pedal sunk down more than usual before the brakes started gripping. I wasn't going all that fast around it and was not really using the brakes all that much. But I was using them more than I would on the street. Is this what is considered brake fade? Right after that turn I slowed down and the brakes worked fine immediately. I took a couple more turns at speed and didn't have any problems.

On a sidenote, holy crap it was fun going around those tight corners. The car handles great.

GlenL

Lilkelt not "brake fade." The term isn't really descriptive. Brake fade is when the brake fluid boils in the calipers. The boiling fluid forces liquid back into the master. With the gas in the lines pressing the pedal gets little resistance and the pedal will go to the floor without slowing the car. Also takes a while for things to cool off so the fluid will condense. A good bleed is needed to make things feel right again.

Your issue sounds more like a master cylinder hiccup. Maybe a leak past a caliper piston. Needs to be looked at.

ninefiveone

It could be a number of things.

Glen's given a great definition of what true brake fade is. Around here and in conversational use, brake fade is sometimes used to describe conditions like you experienced.

I would guess that you've got some air in the lines and old fluid. A good bleed will do wonders.

Scott at Team Harco

In addition to 'brake fade', there are also other scenarios that may have been the cause of what you experienced - one of which is called 'knock back'. It is where the pads/pistons have been forced, or knocked back into the calipers, typically due to warped rotors. It takes more brake pedal travel to bring the pads back into contact with the rotor and permit effective braking. This problem will usually have tell tale signs of brake pedal pulsation if it gets really bad.
Another possible situation I can see from what was described is a brake booster issue. A small crack or loose vacuum connection could reduce the booster efficency. Maybe in a turn this crack/connection is more pronounced than when traveling at moderate speeds in a straight line. Just another thing to consider...

With regard to fade - there are really two primary forms. First is the one described above where water in the brake fluid boils and air bubbles form in the caliper (closest to the heat source). This is caused by three factors:
1) brake fluid is hygroscopic (it will absorb water from the atmosphere).
2) older fluid will have absorbed more moisture.
3) water boils at a much lower temperature than brake fluid.
The end result is that the air bubbles kompress when you step on the brake pedal. This is the soft or mushy feeling you get at the pedal. As mentioned - a good brake bleed with fresh brake fluid will fix this problem.

The other type of fade - though not as common is called 'lining fade'. It does not appear that is what happened here. Lining fade occurs when the friction material gives up its heat dissipation qualities. In other words the friction material gets too hot. Lining fade is characterized by no increase in stopping power with increased pedal effort. The pedal will remain firm - but increased pedal force will not stop the vehicle faster. This type of brake fade is potentially worse than boiling brake fluid. At least with boiled brake fluid, the pedal can often be pumped several times to bring pressure up in the system.

The moral of the story: use good clean brake fluid and a high performance friction material if you want to reduce your risk of brake fade.

pearldrum944

Okay, I understand a lot more now. I replaced the slave cylinder recently when looking for leaks and don't believe I have any. I know my vacuum system isn't in perfect condition, I'll address that when I begin my engine rebuild this winter. I'll also look into replacing my brake fluid. When spinning a rotor, if it is more difficult to turn on one section of it, does that mean the rotor is slightly warped?

Scott at Team Harco

Slave cylinder? The brake system does not have a slave cylinder - only a master cylinder. If it really was a slave cylinder, it was in the clutch hydraulics. Let's assume you mean the brake master cylinder...

That being the case - did you bleed the master? That could easily account for the problem described. Any trapped air in the master cylinder will kompress just as it will elsewhere in the system. And if it was the brake master - why didn't you bleed the whole system at that time? Now is the time to do that, if it hasn't been done.

As for the rotor being more difficult to turn in certain regions of its rotation there are also a couple of possible issues. First is the one you mention - warpage. Other possible reasons include the rotor is not seated flat against the hub (or the hub is bent, or the bearings are bad).

Is the wheel off and the rotor just 'hanging' on the hub when you try this test? Try it again with at least two lug nuts clamping the rotor against the hub. Still have a problem? Remove the rotor and make sure the inside of the rotor hat is clean and free of debris. Verify the mating surface of the hub is also free of debris. Visually inspect the rotor for uneven wear or warpage. Reinstall the rotor on the hub and try the test again. With lug nuts holding the rotor in place use a rigidly held pencil (or similar) to visually gauge run-out of the rotor as you turn it. A dial indicator is the proper tool for the job - but a similar rigidly held object will provide a reasonable assessment.

pearldrum944

I was talking about checking the entire system and the only fluid leak I found was at the slave clyinder so that was changed, and yes it was bled. I changed the bearings recently and am pretty sure the rotor is mounted right. Next time I have the wheel off I'll attempt the run-out test. Thanks.