I have a non-sport, '05 5AT coupe and recently installed a 6/4 BBK. I've noticed that in some braking conditions, I need to tap the pedal first to get a firm brake response. This happens maybe 2% of the time, but its usually after hard acceleration or going up a sudden, steep incline. The brakes feel solid in all other conditions.

Anyway, a shop that suggested I upgrade the master cylinder since my brake system now has to push more fluid. This makes intuitive sense to me since after the initial brake tap, the brakes feel great. Have any of you upgraded your master cylinder after installing a BBK?

From talking to other folks here, reading on stoptech's site, and searching g35driver, I've narrowed it down to either knockback, flex in the master cylinder (a brace can eliminate this), or an under-sized master cylinder which is what I'm asking about now. Any additional feedback is appreciated - thanks guys!

are you sure the system was bleed properly?

If you ever figure this out please PM me & let me know what fixed it. I have everything new(upgraded to brembo's, ceramic pads, drilled/sloted rotors, new stoptech lines, motul fluid) except the mastercylinder and booster and have bleed them till I'm sick of it.........they still feel spongy most of the time and firm very few times. I think it's a vacuum leak to the booster but can't prove it. If they would just act the same way all the time I would be happy, it's the unpredictable pedal I can't stand.

will do and I share your concern.

yeah, I had a reputable shop out here bleed them twice. once with the suction system and once the old fashion way.

i just put in my rotora bbk and i am having the same issue.

i have a blotora, uh rotora bbk as well. what are the specs on your kit?

I don't think changing out the MC is going to solve your problem. Take it from someone who spent nearly $1K trying to diagnose a very similar, if not the same, problem. Street Image spent numerous hours and charged my a grip of $$$ to try and correct the problem. This included changing out the MC. Mind you, my G is an '06 and the MC change took place in '07 so there were very few miles on it. They were taking stabs in the dark at a solution and I'm rather bitter about the whole thing. They suggested perhaps changing to a MC from an '03/'04 Brembo-equipped coupe. I spoke to someone else who had done that and it had not corrected the problem.

My symptom was excessive initial brake pedal travel after hard turns. After researching a while I began talking to some track guys who suggested it might be hub deflection as our hub design is notorious for this. After the initial pedal depression, all subsequent depressions were fine. This suggested pad knock back. I know you can buy some springs to aid with the knock back. If your symptoms are the same, I'd recommend researching the hubs and knock back further.

^100% agreed.

sounds like symptoms of pad knock-back. brake pedal feels kinda mushy the first time you brake after making a turn, but once you tap it, the next braking action is firm.

search pad knockback on google or read some info on stoptech's website. there are some fixes for it, but for a daily driven car, i would simply suggest learning to drive with this knockback.

i've taken the car a few times to the track, and spoken to a few people there. you'll notice that some cars tap their brake pedal right before a braking zone so that when they hit the braking zone, their brake pedal is ready.

im planning to upgrade my brakes to the g37 calipers
i have an 06 g35 coupe 5at with sport package

will i have the same problem??

Thanks Kurtis & Vinh! Like I said, I've read up on knockback, but just want to get feedback on the master cylinder idea since there isn't a good fix for knockback. I've gotten in the habit of tapping the brakes though so I'll live with it for a little while.

Bigc, you ever get a master cylinder installed?

nah, i've stayed with the stock master cylinder. i'm pretty sure i'm just getting a little knockback...only happens after i take a sharp turn.

i have identical issue like yours, i have the megan racing bbk instead, but the situation i get is basically the same.

Do you have aftermarket studs installed on your G bigc?

i do now, but did not when i installed the bbk.