AlwaysInBoost

Last week I did a 2-day event with PCA @ NJMP. Towards the end of the second day, during my 4th session on the Lightning course, I had one of those oh-**** moments. I was pushing the car pretty hard in 5th gear, coming up on turn 1 if you’re familiar with that track, and as I rolled off the throttle to apply the brakes something weird happened. I hit the brake pedal and initially it was firm like it should be, but then a millisecond later it went soft and travelled about ¾ of the way to the floor. It felt almost as if ABS kicked in, but I didn’t feel the pedal “kick back” at all. Then, just as quickly as it went from hard to soft, it got hard again. Braking ability felt the same, just that the pedal was much lower than usual. I managed to slow the car down enough to get thru the turn, but I decided it was best to back off and come in to check things out. I didn’t do any hard threshold braking after that, maybe just 60% or so, but the pedal felt fine for the last lap of that session. The pedal would engage at the top like it usually does and it felt fine the whole drive home too.

Once I got home I put the car in the air and did an inspection. I didn’t see anything out of the ordinary with the hardware. Pads still had about ¼ material left, rotors were still crack free (relatively), my RBF 600 was still clear and the SS lines were fine. The only thing I noticed that was out of sorts was that my brake fluid was on the low side. I usually have the reservoir filled to the MAX line, but for some reason when I flushed my brakes before this event I had it between MIN and MAX. I made a note to get some fresh Motul and top it off but the race shop wasn’t open and it totally slipped my mind.

Has anyone experienced anything similar? For reference I was running PFC-99 pads up front with XP-8’s in the rear on stock C5Z calipers w/SS lines and stock suspension.

The only thing I can come up with is that the low fluid level ‘might’ have sloshed to the front under heavy braking, (with stock 9 year old suspension my car does really nose dive HARD under braking) causing the master cylinder to lose pressure due to the fluid not being by the pickup. This would explain the initial HARD to soft pedal, but not why the pedal got hard again after traveling half way down. Could the ABS pump cycling have induced fluid into the system? If that was the case, though, wouldn’t there be air in the lines then? I’m kind of stumped with this one and don’t really feel like going back out there until I know for sure what the root cause was. Any input would be appreciated.

John Shiels

I seriously doubt it could be the nose dive. If you were coming down a steep hill the nose would be even lower. Being over minimum should have not caused the problem. I think boiled fluid from what you say. 1/4 pads have less insulating capacity so the heat goes to the pistons then the fluid.

From what I remember you having for HP stock C5 caliper capacity is easy to exceed if you are pushing.

I assume you have ducts?

I looked at your profile and 731 HP is time to spend on BBK.

longdaddy

I would guess boiled fluid

Liquid1

Perhaps a large bubble in the brake fluid?

John Shiels

yes probably gas bubble created from boiled fluid. Then it cools and all is fine later on. I can hardly think of a worse feeling if there is no run off.

AlwaysInBoost

yes I have ducts. I really don't think it was boiled fluid though. I've had that happen before and the pedal just goes right to the floor. In this case, the pedal was still rock hard while I was applying pressure, then all of a sudden it went soft and dropped halfway to the floor and just as quickly went hard again. when I boiled the fluid previously the pedal just went to the floor and only came back after it REALLY cooled off.

good point on the part about coming down a steep hill. I didn't even think of that. that section of track I was on was probably inclined if anything, so that rules out my low fluid level/nose dive theory.

wouldn't an air bubble showup earlier in the two days of racing? my pedal was rock hard from the first session to the end... I would think an air bubble would be noticed by a soft pedal at some point throughout the event.

I hear what you're saying about the fluid boiling, and it seems like the logical answer, but I'm pretty sure I'm not boiling my fluid in only 9 session. I'm using the brakes correctly, not riding them, hard pressure on the pedal at first, then quickly trailing off. I'm using rbf 600 and I just changed it 4 days prior to the event. the only use on the fluid was the drive up there (aprox 70 miles, all highway). some of the guys I talked to there only bleed their brakes 2 times a year, and they didn't have any problems besides, that track isn't really that hard on brakes IMO, only 2 spots where you really have to slow down (turn 7 and 1).

John Shiels

bad booster in my truck made the pedal rock hard but it didn't go down or stop well.
You can boil your fluid in one session even brand new if you run hard enough. Thin pads equal heat transfer. i am not familiar with the track. Were they fine when cool agaian?

flink

This is an exact, 100% precisely accurate description of what happened to me when one of the front pads went from "1% remaining" to "0% remaining". What must have happened was that the final thin layer of pad material crumbled away, leaving an air gap. The foot sunk, the backing plate met the rotor and I got brakes back again.

So... you say you have 1/4 pad left - have you actually pulled them out, had a look to see if some major breakage or crumbling has happened?

750 hp? Egads. Your brakes must be superheated plasma

UstaB-GS549

I have had that happen also. It went away and has not returned since. Seeing as I run a lot of "half day" pad sets, I could see a small chunk falling out and the pedal dropping. I have run mine down to the backing plates more than once.

AlwaysInBoost

I didn't actually pull the pads out, just took the wheels off
I hope this is the problem. I will inspect everything more throughly this weekend and report back.

fatbillybob

Interesting problem. There was a racer who lost his brakes in a C6 I think with stoptech calipers. His support staff is deep and I think they run some pro race teams too. He claims the abs went into some kind of "ice" mode. When that happens you got brake but you get no brakes because of the computer action. Then after this brief period things seem to go back mostly to normal. Your statement of it seeming to almost go to abs made me think of this. I think he is petitioning SCCA to allow deleting the stock abs to avoid this safety issue. I do not know how to make the diagnosis but if you think this could be a possiblity contact Oli on this forum. He knows how to contact the driver who experienced this first hand. This was the first time I heard of this problem. Perhaps this problem is more common than appreciated because we racers are misdiagnosing it.

J Ritt

AlwaysInBoost,
Based on all of the info you've provided, I'm leaning towards a diagnosis of some sort of ABS issue. I'm particularly leaning that way because your brakes felt normal more or less immediately after the incident.

A couple of questions: What was the track surface like going into the turn where the incident occurred? Was it bumpy? Did it happen to have any dirt, gravel, or fluid on it? The reason I ask this, is because split mu conditions really throw ABS systems for a loop. A split mu condition would be when two or more of your wheels are on differing surfaces.

The ABS system is designed with a set of parameters it's supposed to "see" or encounter based on your car's capability. It looks at the driver request (line pressure by how hard you're pushing on the pedal), and compares that to how quickly the vehicle is decelerating (using wheel speed sensors). It runs a series of calculations, and then starts tinkering with your line pressures between the master cylinder and each caliper.

When you are in a split mu condition, the calculations become a lot more complex. On asphalt grip is very high. On ice it is very low. On gravel it's higher then you would think, because the tire can dig in. The list goes on, and the car is programmed with reactions for when it thinks it 'sees' these varying conditions. With split mu, the car is essentially getting data from multiple wheel speed sensors that doesn't add up or 'make sense.' The car then struggles to interpret that data and react accordingly. What if the system thinks you are on concrete, but you're really on ice? The result is that your ABS system does some funky things like what you describe.

I've had some weird ABS things happen in my C5Z on the track as well, and it's happened where gravel had been dragged on the track, someone had dumped fluid, I was floating over ripples/bumps in the brake zone, etc. What has worked for me is immediately and completely letting off the brakes, and then quickly hopping back on them again. Basically,you're hitting the reset button, and allowing the system to start interpreting a fresh set of data.

All of that said, 750hp on just about any track is a recipe for brake punishment. Your car will put massive energy into the brakes with that much power. You may want to get some rotor paint or caliper stickers just to see what temps your various components are running at...then modify accordingly. That will really help tell you what you're dealing with, how much you need to spend, etc.