This is odd to me, but...

Hard on the brakes, the car grabs, then lunges forward and feels like there's no brakes. Then it eventually stops. :dunno

Axxis Ultimates, stock calipers (rebuilt), and stock rotors. >60% pad life left and all were bedded in properly. This just started Saturday.

Any ideas?

This is odd to me, but...

Hard on the brakes, the car grabs, then lunges forward and feels like there's no brakes. Then it eventually stops. :dunno

Axxis Ultimates, stock calipers (rebuilt), and stock rotors. >60% pad life left and all were bedded in properly. This just started Saturday.

Any ideas?

could it be when the ABS kicks in?

I wasn't getting into the ABS yesterday at all until the stop box.

Fluid/oil spewing onto the rotors?

Nope, checked yesterday. I really don't think I was boiling the fluid either.

Master cylinder vacuum leak??

Does the pedal go to the floor?

No pedal is the same. I'm replacing the pads this week with Hawk's, but I don't think it's the pads.

vacuum booster. must be vacuum leak somewhere, possibly the gasket between m/c and booster. or else the booster is bad.

vacuum assist has enough boost to work initially to give you some bite, then assist fades off, resulting in less actual braking even though you may be pushing the pedal harder.

very interesting feeling going into turn one at vir at max tilt boogie thinking you are about to outbrake someone when these symtoms show up....

marshall
just replaced vacuum booster on race car

how can you be sure its a bad booster vs. just a vacuum leak. any reason to suspect a bad master cylinder?

i'm curious as my brake feel has been less than ideal lately when autoxing... not sure if its operator error or something 'real'.

Kent

vacuum booster problem is a vacuum leak.
other vacuum leak options are the 6in hose from manifold to booster or the check valve on the booster.

bad master cyl would have the opposite symptoms, soft initial feel that gets better with repeated pumping (or not!). a bad master will also not hold pressure if you sit there stopped with the engine off and your foot on the brake.

very interesting feeling going into turn one at vir at max tilt boogie thinking you are about to outbrake someone when these symtoms show up....

Hated that feeling...drove us to going to a non-boosted setup in the CMod car.

Sorry to bring this thread back from the dead but I'm having a similar problem. Braking down from ~110 to 35 and lost brake boost. Did a couple more laps after that but problem didn't reoccur. From the discription above by mlytle, it sounds like a vacuum problem. It only occured once. I've had my mechanic go over the car and he can't find a problem. How do I figure out if vacuum is the problem? TIA

I don't know of any good way to determine whether the problem is in the booster or external to it. But what I'd do is to replace the vacuum line and check valve to the booster, make sure that there aren't any other bad hoses on the intake, and see if the problem reoccurs. if it does, replace the booster.

thejlevie, thanks. Let's say I do loose vacuum, will I still be able to stop the car or
will I complete loose the brakes? I could not reproduce the problem on the street and am planning on taking the car to the track this weekend.

If the problem is the booster you'll still have brakes. It will just require greater pedal force to achieve the same braking. You can find out what that's like by disconnecting the vacuum line from the booster and plugging the line.

Thanks again. I'll make sure I stay well back from the cars ahead until I'm confident the problem won't reappear. Let's say the problem does reappear, is it best to just stand on the brakes or should I try pumping the brakes to 'build up vacuum'?

Hated that feeling...drove us to going to a non-boosted setup in the CMod car.

+1.....unboosted brakes FTW :alright

Thanks again. I'll make sure I stay well back from the cars ahead until I'm confident the problem won't reappear.

Uh, I'd be more worried about the car right behind you who is not expecting you to jump on the brakes 50' early. :eek: :eyecrazy :mad

Uh, I'd be more worried about the car right behind you who is not expecting you to jump on the brakes 50' early. :eek: :eyecrazy :mad

In a DE situation, I would expect most folks to leave plenty of room in the braking zones anyway since passes are only allowed on the straights. There's no advantage to late braking and sucking up to the back of the car in front of you.

Uh, I'd be more worried about the car right behind you who is not expecting you to jump on the brakes 50' early. :eek: :eyecrazy :mad

Good point... I'll keep one eye looking forward and one eye on the mirror and ...
I'll be definitely keeping some clean track around me until I've got some confidence in the brakes. It's not too hard at a Chin event with > 3 hours of track time per day :)

Well, they should leave room. But I've seen some pretty boneheaded driving by folks that should (based on their experience level) know better.

In a DE situation, I would expect most folks to leave plenty of room in the braking zones anyway since passes are only allowed on the straights. There's no advantage to late braking and sucking up to the back of the car in front of you.

Well, yes and no. Depends upon the run group. In upper groups (i.e. advanced or instructor), the only way to get past a low horsepower race-suspension car (e.g. E36 race car) in my street/track suspension heavy M Coupe is to close up on him on the straight / braking zone. Otherwise he'll carry so much momentum / speed thru the turn that I won't catch him again until he's way down the next straight. OTOH, I obviously have to leave room to setup a heavy slow-turning high hp car. Last, I generally know who is driving what. If I follow RayDoc into a turn, I know that he will brake about where I would, and I count on that. That's also the only way I'm going to get past a fairly equal car. If I'm following, say, a Mustang with relatively poor brakes, I leave him extra room. Obviously we're not inches from a bumper, but I would certainly be surprised if the car in front of me braked a couple of markers early.

And, yes, in novice/intermediate groups, we try not to get too close. Still, they may need to fill the mirror of the car in front in order to induce the point-by.

My point stands -- if I were unsure of my brakes and were anticipating braking early, I make sure there weren't a car right on my bumper going into the braking zone. I'd so the same on the street, too.