So here's the problem I've been having. It's not so much and issue on the street where speeds are normal (it still happens though) but it is an issue on the track where reliable braking is a must.

On the first push of the brakes the pedal feels soft, not so much spongy, but not firm like I would like. I don't know when it started happening, for all I know it could have been like that since I bought the car. I first noticed it when I took the G-Coupe in to Infiniti and got a loaner car. The Loaner brakes grabbed instantly, it took me a few stop signs to get used to the feel (I had got a loaner twice, both sedans one an '07 another an '06).

After a while, in my G again, I for some reason started to pulse the brake once and the brake would feel nice and firm like it should. So I do this going into every corner on the track now.

Is there a solution to this?

The brakes are essentially stock, with Motul 600 fluid in the lines. Flushed every day when I goto the track.

Oh, and the track shop that fixed up my brakes one day and flushed the lines made a comment that the brakes felt reel soft but they shrugged it off because they were power brakes.

 

my guess is that you got some air in the lines. Bleed the lines again. If that still does not solve the problem you might have air trapped in the ABS module when you changed out the fluid. I'm not positive but I believe you need to take the car in to have the ABS system bled as needs to be hooked up to a computer to cycle the ABS system.

 

The pedal on my 04 6MT sedan is also soft and inconsistent much as you describe. Brakes have been bled several times (plus a new MC) to no effect, so it's not air in the lines on my car. Best I can reckon, I believe the pads are getting pushed back a little bit by surface irregularity, vibration, or runout in the rotor, which creates slack in the pedal. A quick pump or two before braking sets the pads, calipers, and pistons back in position, removes the slack, after which the pedal feels more firm and engages higher. I don't know how to fix it, although I think ss braided lines, a master cylinder brace, and a larger bore master cylinder (if one is available) would help brake pedal feel overall.

 

Well someone had suggested that to me. But the car doesn't need to be moving to make the pedal feel soft again for me.

Humm, I'll defiantly have to ask the track shop guys about that net time I go. Because I sure as hell know it shouldn't have air in the lines from all the bleeding I do to it.

 

You can also "bleed" the ABS module by doing a couple of very hard stops with ABS engaging, i.e. like on a slick surface. As long as you can get the ABS unit cycling, you should be able to push the air out of the module to the calipers, at which point you can re-bleed the calipers.

 

My '06 is doing this as well. It's not a bleed problem but more like an adjustment problem. On late '70's and early '80's GM 4 wheel disc cars, this was a problem as the rear calipers would adjust with use of the parking brake. The pistons 'screwed' out to set the parking brake but people didn't konw that and the 'screw' would sieze. On rear drums, if you were out of adjustment you would have a low first pedal and a firmer second because you pushed the shoes had to travel further with more fluid.

 

Very good suggestions on the last two replies. I guess you could see how your rear pads are. If they are low, that might be the cause of the pedal travel.

 

Makes sense, except in our cars, the parking brake is cable activated, and is a separate system. But, you may be on to something - the pad wear could indeed be an issue, as you may have to compress the pedal a bit for the pistons to make contact with pads that are worn a bit. This is called "knockback":

http://www.stoptech.com/tech_info/wp_knockback.shtml

 

The rear parking brake is actually a drum brake inside the rotor. I have a picture if you really want proof.

Usually after a day of track use I replace the brake pads completely. So far I'm pulling for the ABS idea because a) I know the least about it and b) it's the most costly/time consuming to fix. Not to mention it'll happen if I'm just sitting in the garage not moving. I'll pump and they'll get firm then wait a second or two and on the first push they are soft again.

Regardless thats a good knockback writeup in that link. Someone had suggested that to me at the track.

 

Yes, I know it the parking brake is a drum brake inside the rotor hat, but it is not hydraulically activated via the main brake system. It is activated by a pull cable, so it should have no effect on pedal feel.

 

Yes, I am aware of the parking brake system because it was used in the '68-81 Vettes. What I didn't konw was we had that type of system on our cars. I will have to read the link you provided on the 'knockback' and see what I can come up with. I didn't know it had a name.

But you are leaning towards the ABS as the problem? I'll try your suggestion this weekend and let you know if it works. I want to fix the problem before I add drilled/slotted with stainless lines and Motul fluid.

 

I feel the same way about my '04 sedan, soft brakes that don't stop well enough and even pretty much fail to activate ABS on good dry surfaces in warm weather because they just don't have enough stopping power even when I stand on them like crazy!

I installed stainless steel lines, Hawk HPS pads, slotted rotors and Wilwood Racing fluid and I did not get much improvement. I bled them thoroughly and then had them re-bled by a shop with a vacuum system just in case I hadn't done the job well enough (they didn't get any bubbles).

I find they just aren't very good brakes. I'm about to try HP Plus pads and fresh fluid again before a track event this Friday. Maybe they'll perform a bit better but I'm becoming resigned to the idea that I really will need a BBK to solve. Mike at Riverside's going to have to hook me up with a G37 system! Only problem with that is (besides costing over $2k) is that there are no aftermarket performance pads or rotors available for them yet.

 

so, did the abs suggestion solve the issue?

 

I'd also look into a faulty brake master cylinder

 

Good luck finding a cure for this. I have an 05 with new rotors, pads, ss lines, motul fluid that has been bleed 6 times and I still have a soft pedal that is unpredictable. I think it's a booster vacuum issue but I can't prove it, as there are no visible leaks in the system with leak check soap/solution. I don't think proper booster vaccum is there on the first pedal push, but once you put the demand on it it gets there by the second or third. I haven't put a vac gauge inline with it as I don't have one handy to see what kind of pressure is there. I think it's either a design flaw in the booster that no one at the factory is concerned about, hence no TSB, or the booster line has insufficient vacuum in the stock configuratuion. It's got to be something design wise as this is a widespread problem.