Shikaitsu

Quick Summary: Can brake fade occur from extended downtime

I had asked about a braking issue I developed about two years ago when I let me 2005 C6 sit over the winter while I was installing a twin turbo kit. Back then I thought it was a vacuum or brake booster related issue, but I have now ruled out both of those as possible issues. The car generates plenty of vacuum both at idle and normal driving, and I had the brake booster replaced but the issue persists. The brake pedal is very stiff, to the point where I almost have to stand on it to stop and if I let off just the slightest amount of pressure on an incline the car begins to roll (the brake lights are still on when this happens). I have been dealing with the issue for the last couple of years by giving myself more braking distance (I do not drive this car daily) but now that I have finished my turbo build I'm addressing other issues like tires and stopping power.
To clarify, the setup is OE Z06 calipers 6 piston front 4 piston rear with AC Delco drilled rotors and EBC Yellow Stuff brake pads, all installed about 3 years (less than 10,000 miles) ago. I am using SS brake lines and the OEM master cylinder and brake booster (all of which worked fine when I swapped the brake system so I don't think that's related) with a brake flush twice a year using Synthetic DOT 4 because I use the car for road course racing and mountain trips. There is always the possibility that this issue has progressed over time and I just didn't notice until I drove the car for the first time in 3 months but I've never experienced brake fade and I'm not sure if that's my issue.

Dano523

So is brake fade coming from the pads getting hotter then designed to be run/working boiling temp of the brake fluid not high enough, or is there a problem in the lines instead. The yellow stuff pads are only designed for up to 900*, and really an autocross pad once warmed up, and not a high speed road course pad isntead. Same goes for some basic Dot fluids, with not a high enough boiling point as well.

Also, when you did power bleed the brake fluid to swap it out (which should be happening after every week end road course uses since you are getting the higher boiling point fluid close to it boiling point and it will pull more humidity out of the air), did you use a tech II to cycle the abs pump to get any air/old fluid out of it during the power bleeds?

As for brake getting a little funky from sitting for a long period, and no leaking at the calibers/lines or ABS unit/ all the fluid has been power flushed out and new fluid in (including the ABS pump), then would suspect that the master cylinder plunge seal is giving up the ghost, and it's time for a new master cylinder.
Hence driving around for a while will warm up/flex the plunger rubber to make is seal a touch better, but when not driving, the plunger rubber will shrink back down, and when you go to press the pedal, will have more pedal travel until the U rubber plunger swells up from pressure to seal against the cylinder walls correctly. Also, if you are sure that there is not leaking/no air in the lines or pump, and you hold the pedal down for a while and the pedal slowly works down farther with the motor off, then master cylinder plunge seal is allowing fluid to pass by it. If master cylinder is holding, but get funky with the motor on, then suspect the booster isntead.

Once last thing, and although the Yellow pads claim to be able to work at cold temps, they really don't start to bite well until they get up to around 200*. So street driving, you don't get the pads up to temp enough to have a good bite, and the slight lift off the pedal and car rolling, it about right. Hence want to run a street pad on the streets that does bite well through cold to around 600* working temps, then pull those pads and install race pads that will have a higher working temp for road course work (but will still need to put in a slower first lap until the race pads and rotors do come up to heat to bite correctly).

Hence running pads like DT-70's with their high working temps, pads below 400*, you can stand on the pedal with both feet, and the pads will stop the car about as well as just opening the door to drag your foot on the tarmac instead.

Shikaitsu

A lot of great information there, I did not use the Tech UI module as I don't have access to but I work for Maserati so I use the ranger method and a hydraulic fluid exchanger until the fluid comes out golden. As for pedal characteristics, it remains firm under all conditions and the braking characteristics don't change under any circumstances. I did an HPDE while the condition was present (not the brightest idea) and even letting the brakes get warm and taking a few soft laps in Novice class they never seem to get better. I hadn't considered the master cylinder because I always thought master cylinder failures resulted in a spongy or low pedal but this seems to stay at the top 30% of the pedal range and incredibly firm and there is no difference in how the pedal feels if the engine is running or not.

Dano523

Write it off as the brake pads in play the problem alone,
What they claim since they call them a race pad as well, but never really bite hard enough through-out what is called as their working heat range.



Hawks on the other hand, where their race pads once up to temp, will bite hard enough to leave seat belt welts in your passengers chest from hard braking if you are running race slick to get that kind of ground grip under braking.
Note here, if you are not running at least summer performance tires to begin with, then all the braking grip in the world is not going to make a difference, since you will get into the ABS system too quick.



As for ABS, really needs to be cycled to get the old fluid out of it during a brake fluid power bleed. Not sure what devices you have around the shop, but would check since one of them may be able to work on a C6 to do the automated break bleed that cycles the pump to get the old fluid out of it.

And last one, if you are running HPDE, run the car at least in Comp mode, and if you have the talent, then just turn off active handling all together. "All Nannies On" into a slower corner under hard braking, will end up with AH kicking in way too much and the car/AH fighting you to keep the car going straight, instead of turning into the corner. Hence the fasterest way to end up off the track braking hard in to a corner.

In comp mode, you can let the back end get out a little more, come into a corner way hotter under breaking before AH tries to save you instead.

All nannies off, only ABS is active, and you can really hang the rear end out, and so much that rear end can over take the front end without out AH kicking in on you.

sccaGT1racer

What your descibing here is not brake fade. You either have a power booster issue,vaccuum loss, collapsed/restricted line,too high of heat range pad or too big of a master cylinder bore.

Red06Vette

What's your vacuum at the power brake booster?

Chatman

This is something of a hijack but does concern the Hawk graph. I got my pads 2nd hand. They say Hawk DTC and say HB532. Which Hawk DTC are they?

Shikaitsu

Are you asking me to check vacuum on the booster or on the line coming from the back of the intake going to the check valve?

drewz06

toss some hawk dtc-70's on there and try to reproduce....i'm betting you'll have perfect breaking... just my two cents... you're gonna need em eventually....

drewz06

theyre not dtc... dtc-70 will show clearly on the pad...

googled hb532 -

tommyc6z06

I agree...kinda. another car, but after turbocharging I had no 'power assist' when suddenly coming off the the throttle (boost). It was rear engined, with booster in front.
I added a one way check valve into the vacuum line going to the booster. That 'almost' cured it. But, I still had to add another, larger vacuum reservoir (made of 4" PVC pipe, and capped). Good luck

Shikaitsu

Looks like one way or another I'm gonna have to replace the brake pads, probably keep two separate sets which is slightly annoying but that's the cost of doing business. Do you keep a separate set of rotors as well or use the same ones on and off track?

drewz06

yeah i was planning on using stock rotors with hawk hps+ for street with mp4s.... and r7's with girodisc and dtc70s.... hate the stock calipers, really want to get the ap racing set so i dont have to pull the caliper every time....

after the last event before i parked it for the rebuild, i just left the dtc70's on ... use solids or slotted for the track... drilled crack faster...

Red06Vette

Check the line from the intake to the boostercheck valve.

Chatman

FYI
Hawk GTC
HB532G.570
appear to be 60s