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Brakes spongy unless released & quickly reapplied

5K views 38 replies 14 participants last post by  Supercharged 
#1 ·
On my recently purchased 2009 G9 GT with what I think are stock pads and rotors, I've noticed the pedal feels kind of spongy compared to most cars. The brakes work great, just don't like the pedal feel. THe other day I discovered that if I release the brakes and quickly reapply them, the pedal is high and firm like I'd expect them to be.

On every other car I've ever experienced, this is caused by one of two things: Air in the lines, or pad knockback, which is what happens when axle or bearing runout/endplay cause the pads to be pushed back into the calipers as the wheels rotate, and that slack then needs to be taken up before they really work, causing a long pedal travel.

I don't believe it's knockback, because on a car that doesn't have solid axles, it takes a lot of bearing runout to cause it, and I don't have any other symptoms of the sort.

That leaves air in the lines. I bled all 4 corners on Saturday, and the fluid looked good and I didn't see any significant air coming through the clear bleeder line. Nonetheless, I was suprised that the situation didn't improve at all.

SO...is this spongy pedal a normal characteristic of the G8? If not, any ideas where I should look next?
 
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#19 ·
You probably fixed this by bleeding but fluid with alot of moisture might cause this.
I figure moisture wouldn't be an issue until the fluid got quite hot (air resulting from the lower boiling point of the moisture-rich fluid).

Don't think you can change the pedal travel notably by doing simple stuff like the stainless braided brake hoses that many people like to do.
Stainless lines might help the feel a little. While it might not solve the whole problem, it likely wouldn't hurt anything.

It's like taking a 16-bit audio signal but only using 1-2 bits of that, and leaving the other 14-15 bits there to do nothing. If you're trying to record music, it will sound like crap because of super high distortion because the recording didn't take advantage of the precision of using all 16 bits.
To use an audio/bit example... think about the frequency range. If I'm recording something with minimal base, but lots of distinct treble tones (ie. a flue solo)… I'd rather use my bits of resolution to focus on the treble than full spectrum of frequencies. Yes, there would be additional distortion at the lower frequencies… but I'm sacrificing that for the increased resolution in the more desirable range.

Applied to the brakes… I'd rather have less resolution at lower braking levels (0%-25%) if it results in greater resolution… and greater precision… at higher braking levels (25%-100%).

"short travel" does not equate to "binary brakes" unless the driver is incapable of modulating how much pressure they apply to the pedal.
Agreed 100%. When I was looking into brake pads for the track I was a little surprised to see "for experienced drivers" listed at a caution on some pads. After reading more about initial bite it became more obvious... the high initial bite produces a greater stopping force, but it takes an experienced driver to properly modulate the pressures. Based on your experience, and description of the current feeling, it sounds like you're more comfortable with the greater stopping force by a braking setup with high initial bite with minimal initial peddle travel (as I would describe as having a "firm" peddle) than one with less initial bite and more peddle travel (as I would describe as having a "soft" or "squishy" peddle).

I just think it's really strange that releasing and immediately re-applying the brake makes them feel like they should. I'm deeply familiar with how braking systems and their boosters work, but that's a head-scratcher to me.
Agreed… it's almost like you're having to rebuild pressure. Almost like a "slow leak" (I don't know if that's even possible though) more so than lines, fluid, etc.

I wonder if you flushed your fluid and changed to stainless lines if that would improve things enough? Then again, at that point you're part way into the labor of swapping calipers.

-Todd...
 
#3 ·
Yes, it's normal on these cars. Like you said, the brakes are good just a little too much play for my liking. Swapping pads will help. I'm doing a full upgrade in the spring once the stocks wear out.
 
#4 · (Edited)
Assuming it's normal as many people have this complaint about GM cars when coming from other brands. While many people make the claim, GM brakes aren't spongy. Brakes of other brands are binary. They set them up to lock up your wheels with 0.000000000000000007mm of pedal travel. Why? I don't understand why people like that. Why would I want my brake pedal to have no usable travel? And why do so many people call brakes that actually utilize the pedal travel "spongy"? The opposite of "stiff" (as in a stiff pedal) is not "spongy."

You'll get used to it. It gives you more precision anyway.
 
#9 ·
Assuming it's normal as many people have this complaint about GM cars when coming from other brands. While many people make the claim, GM brakes aren't spongy. Brakes of other brands are binary. They set them up to lock up your wheels with 0.000000000000000007mm of pedal travel. Why? I don't understand why people like that. Why would I want my brake pedal to have no usable travel? And why do so many people call brakes that actually utilize the pedal travel "spongy"? The opposite of "stiff" (as in a stiff pedal) is not "spongy."
And a stiff pedal doesn't necessarily correlate to higher effort. Making changes to any given braking system that reduce pedal effort always means you get longer pedal effort in the tradeoff, but that is not to say that short-travel systems, designed to be that way, are necessarily high effort. In most cases, they're not high effort, they're just short-travel.

You'll get used to it. It gives you more precision anyway.
No, I won't, and the precision argument is not true. Unless we're talking air brakes, precision comes from brake pedal pressure modulation, not brake pedal travel modulation. And believe me, running a car on road courses for 10 years without ABS and autocrossing it for 5 years before that has taught me an awful lot about brake modulation with a high, firm pedal.
 
#10 ·
No, I won't, and the precision argument is not true. Unless we're talking air brakes, precision comes from brake pedal pressure modulation, not brake pedal travel modulation. And believe me, running a car on road courses for 10 years without ABS and autocrossing it for 5 years before that has taught me an awful lot about brake modulation with a high, firm pedal.
It is true - you're looking at it the wrong way. Both travel and pressure matter. The point is that with longer travel, it takes less precision with the foot movements to get the point you want, because the pedal action is essentially "stretched out" so your movements are longer and require less precision. I can't really explain it that well. It's like taking a 16-bit audio signal but only using 1-2 bits of that, and leaving the other 14-15 bits there to do nothing. If you're trying to record music, it will sound like crap because of super high distortion because the recording didn't take advantage of the precision of using all 16 bits. That's probably the best way I can explain my point at this time.

I'm not saying that I am unable to drive a car with binary brakes that lock your wheels up when you even think about touching the brakes. I just don't see why anyone would like that. And I don't see what the problem is with the GT brake pedal travel. It's longer than with many other cars. And? Are you too short that you barely reach the pedal? Are you so weak that you can't press it fast enough?

My argument is that the longer pedal travel is easier to use, and that it still works fine for whatever purpose.

My mom's Accord - which I can drive just fine for the record - has WAY too 'immediate' a brake pedal that accomplishes absolutely nothing worthwhile. Not a single reason for that crap. I don't actually want the pedal to travel all the way to the floor or anything, but I don't want it to be anywhere near as immediate as it is on her car any more than I want the gas pedal to be that immediate. Though her car has the opposite problem with the gas pedal - it doesn't feel like it's connected to anything, because the car is too damn slow. haha :D
 
#5 ·
Following this thread...mine have started to do this also and with the same solution to get peddle feel back (release and quickly reapply)...Had the car since new and don't remember it doing it before even with the first two sets of pads...
 
#6 ·
I also have this. I don't remember having it before I put new pads and rotors on the car two months ago. I didn't think much of it because the breaks do work fine. I just thought it was because I went with a cheaper solution than OEM.
 
#7 ·
OEM brakes are great IMO. It's not a spongy feeling, just too much travel. After a couple of panic stops, I want more initial bite and better feel. That's why I'm upgrading. Not that the stocks are bad, but aftermarket is usually better. My .02 cents.
 
#8 ·
The GT brakes are spongy, at least in my opinion (though I'm not the only one). It was something on my list that made me glad I got the GXP, which has greatly improved pedal feel over the GT.

To the best of my knowledge, the GXP and GT share the same brake lines, master cylinder, and ABS unit. The difference is the front Brembo calipers on the GXP. The downside is that they don't fit behind the GT 19" or 18" wheels, though I think they can be made to fit with spacers.

Glancing at your signature, if you ever plan to take your car out on a road course, I'm guessing Brembos would be an upgrade you could appreciate. You can get in touch with I12XLR8 here on the board for his experience running Brembos with his stock GT wheels.
 
#14 ·
The GT brakes are spongy, at least in my opinion (though I'm not the only one).
To quote the source:

Engineering Editor Jason Kavanagh said:
For all the things done right in the 2008 Pontiac G8, the "Buick-circa-1988" squooshy brake pedal is made that much more incongruous. The bite over the initial inch or so of pedal travel is largely nonexistent. It's like stepping on a Care Bear.
Exactly what I'm talking about.

I just think it's really strange that releasing and immediately re-applying the brake makes them feel like they should. I'm deeply familiar with how braking systems and their boosters work, but that's a head-scratcher to me.
 
#11 ·
You're each talking about two different things: travel and effort, and they're each distinct. Travel is the distance the pedal is able to move, and effort is the amount of force it takes to produce that travel.

MFE isn't talking about a pedal with no travel. He's talking specifically about the failure of the GT brake pedal to produce a roughly linear increase in resistance to travel - a roughly linear increase in effort, in otherwords - the further into the stroke you press the pedal.

So the issue isn't that the GT pedal travels too far. It doesn't. The issue is that it doesn't press back and, as such, gives you a rather vague and poor sense of how deep into the stroke you are.

Coming from a Mazda6, I test drove a G8 GT and immediately noticed the difference in pedal feel. Got to love Mazdas...
 
#12 ·
So the issue isn't that the GT pedal travels too far. It doesn't. The issue is that it doesn't press back and, as such, gives you a rather vague and poor sense of how deep into the stroke you are.
I just can't duplicate that feeling. I dunno man. I've never once been unsure of what was going on with my brakes.

When comparing, to me - well, I will give another audio analogy. You know how many people tend to confuse volume with sound quality, i.e. if you A/B them the same signal at slightly different volumes, most people will assume the louder signal to sound better? You turned up the volume with your Mazda 6, and in the G8 you turned it back down. It may seem lower quality, but it's just different.

That's what it's like to me, anyway.

Anyway, I have probably explained what I meant well enough now, so I'll leave the discussion alone.

Edit: Oh, and one more thing. Don't think you can change the pedal travel notably by doing simple stuff like the stainless braided brake hoses that many people like to do. You'll need to really dive into the brake system if you want to really change it. Assuming we're still talking about a car that MFE just doesn't like the brake pedal feel of (as opposed to it truly having air in the lines).
 
#13 ·
Modulation does not have to come from long pedal travel, and "short travel" does not equate to "binary brakes" unless the driver is incapable of modulating how much pressure they apply to the pedal. If that's the case, they're probably a paraplegic using hand controls anyway, and the argument is moot.:D

I like to modulate pressure, not travel, not only because it feels better to me, but because I'm less likely to accidentally catch the gas while braking hard with such a long pedal stroke. If this is considered normal for these cars then I'll have to add it to the very short list of things I don't like about it. I don't *like* a pedal that feels like the brakes need bleeding, and this is one of very few cars I've encountered in almost 30 years of driving that feel that way.
 
#17 ·
GM stock pad tend to be very quiet and very clean. The GT has no brake dust, but the down fall is a spongy pedal. You could get a set of nice dust stoptech pads, hawk HP (not HPS) or something better, with some nice black wheel dust, and the pedal will feel better. Actually stock Brembo/GXP pads feel stronger and less squishy then most other pads I have ever had.

I have a GXP and the stock Brembo pads are very dusty and firm feeling. When switched to Stoptech front pads, they were almost as dusty, but initial bite and the firm pedal were somewhat lost. I think the GM-Brembo pads are Ferrodo... not sure what model though.

I hate squishy pads.
 
#20 ·
I agree the GT brakes feel rather spongy and provide poor modulation. Even with new brake pads, rotors and fluid flush. Coming to a quick stop often ends up my head jerking forwarding from applying too much pressure. They are just tricky to modulate.

I find my GTO brakes are less spongy and easier to modulate.

As far as stopping power is concerned I don't know which are more powerful, the GT or the GTO brakes.
 
#22 ·
Same master cylinders. Is there any adjustment between the rod connecting the brake pedal and the booster?
 
#25 ·
There is in some cars, but I don't think that's the issue here. It's not a matter of free pedal travel before the brakes activate, like what happens if the booster rod is too short, it's just....mushy.
 
#23 ·
Just as a general rule. with all other things equal (brake MS, fluid lines etc)

Brake force is dictated by total piston area and how far off the center of the rotor (ie.. rotor size)

Brake feel is dictated by the # of pistons in the caliper. This is why multi-piston calipers have better modulation.

I'd GUESS since the MS is the same, the total piston area of the GT vs GXP isn't that different. But the force is greater w/ the bigger rotors and feel via the multi-piston setup
 
#27 ·
Brake feel is dictated by the # of pistons in the caliper. This is why multi-piston calipers have better modulation.
Just in the interest of accuracy, the number of pistons doesn't contribute directly to brake feel, or to braking power, as some people believe. Brake feel is first dictated by the ratio between master cylinder piston area and caliper piston area, coupled with how much boost is applied if they're power brakes. If you increase the size of the master cylinder and hold all else equal, you increase the pedal effort but shorten the required pedal travel. If you decrease the size of the m/c piston, you reduce pedal effort, but lengthen the stroke.

It is true that 4 and 6 calipers with designs like Brembos tend to have a very stiff caliper, and that in itself improves pedal feel, whereas single and dual-piston calipers tend to be made of less rigid designs, which add compliance to the system and degrade pedal feel.

The real reason for 4 and 6 and x-piston calipers is that they can pack a lot of piston area into a very shallow design, meaning they don't stick out very far past the outer diameter of the rotor compared to a single/dual piston design of the same piston area. The reason that's important is it means you can package a lot larger rotor under any given size wheel than you'd otherwise be able to do with single or dual-piston caliper.
 
#24 ·
I came from a Grand Prix GXP with Brembo brakes and when I test drove a G8 I though there was something wrong as the brakes were so soft. (They seem to go almost to the floor)

The brakes are the only thing I miss from the GP GXP. Except maybe that it was impossible to tell when the DOD (AFM) switched for 8 to 4 cylinders
 
#28 ·
I don't think there is a big difference in the total piston area for the single vs the oem BBK kits available. Why? My theory is if the total piston area was much larger with the BBK, brake travel and effort would greatly increase because most are stuck with the oem master cylinder.

So if the total piston areas aren't changing much, then my theory is that the # of pistons is helping with modulation. Especially if they have different sized pistons. I'd also theorize that having a longer caliper that has multi-pistons would allow you to run a pad with more area for friction. spreading out the load and heat
 
#29 ·
Wow, this is a very informative thread. Ultimately, though, what is the ideal solution for a shorter pedal travel and strong "bite" with small effort? Like many others here, I have been in some cars that "feel" like they have "better" brakes due simply to pedal travel. My wife's 2009 BMW 328xi coupe, for example, has super-touchy pedal feel compared to our G8s.

I'm running stainless steel lines with the GXP front Brembo calipers. What's the next step, if any, to achieve the feel I'm describing above on my wife's BMW?
 
#33 ·
Use the stock gxp/brembo/SS pads. I tried stop tech front pads, but they dust almost as much and pedal travel has increased. So I just ordered some OEM pads from rock auto. The box say ac delco, gm, brembo and made in Italy... Awesome :)
Hopefully the bite goes back to what it was with the new OEM brembo rotors and pads.
 
#35 ·
Race pads or Hawk HP+. Noiser, dusty, more hot bite, ?? cold bite.

Wow, I thought the GXP stock pads were a little too touchy with not enough feel at the same time. That is why I tried the Stop-Techs. The Tech's seem great some days and others don't have any initial bite. Also I am pressing down further with the Stop-Techs, and brown dust is not much better than black dust. So back to stock for me.

The GXP stock front Brembo pads come stock on Evo's, Camaros, CTS-V's, and many other brand performance vehicles. They are every bit as aggressive as BMW's as far as I can tell. The front tire dust certainly looks the same as older BMW's. I've driven late model M3's and 328's. Any more aggressive would tend to lock the front wheels up to easily, unless you have some really good rubber. This can be a problem on snow tires and lesser rubber.

Are you sure you don't have some air in the system??
 
#37 ·
I'm not an expert here but don't you have to consider the difference in weight and momentum when stopping a 3 series vs the size and weight if the G8?

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#38 ·
I am also not an expert which is why I'm here seeking some expert advice! And, judging from a lot of the science being thrown around in here, most are much closer to experts than myself!:driving:

Absolutely, weight inconsistency makes a 3-series coupe perhaps not the best comparison. However, I have noticed the same "feel" when test driving a 2012 535xi (don't worry, I'm not jumping ship) and driving my in-laws' new diesel X5. It seems to be a "BMW thing" as far as pedal feel. Since we've established the G8 brakes quite well, I'm not looking for better brake "performance", per se (although that is always welcome) I'm just looking for a tighter-feeling pedal response.
 
#39 ·
I will be doing 04-07 V calipers with ceramic pads this spring when it warms up. Got the calipers already and getting less expensive pads and rotors from eBay ordered soon. Curious how much better braking will be plus it will look awesome behind the Camaro SS 20's.

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