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clydesdale

Recipe for cooking brakes...

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clydesdale

I'm troubleshooting brake problems and I thought I'd throw this out to the forum to get some opinions. The basic problem is that I'm losing the pedal anywhere from 8 minutes to 15 minutes into a race when doing hard laps (about 1-2 seconds slower than qualifying laps). When I say "losing" I mean it goes to the floor-- the most obvious reason being boiled fluid.

 

Here are the details. Last year I ran both RBF600 and SRF and had the problem either way. I've got a 2005 STI at 3100# comp weight and around 270hp using the stock Brembo calipers and various rotors both one and two piece. Pads are DTC-60. I have 3" brake ducts. Thinking that better brakes would be a step in the right direction I switched to the Stoptech ST40 and ST22 calipers this year and went to a mixed compound DTC-60 front and DTC-70 rear. First hard session on track this year and I get the same damn problem, pedal goes to the floor after 5 laps. There is nothing quite like racing to make you feel like you've just wasted a great deal of money.

 

I have a couple of ideas about what could cause the boiled fluid (if that's actually what it is), but any feedback or opinions are welcome at this point. Since there are only three components unchanged it could be one of them: ABS, master cylinder, or brake lines. The master cylinder seems the most likely of the three, the possibilities being not enough free play in the pedal connection, a faulty plunger, some other internal component, or maybe the booster.

 

There are no leaks. There are no obvious kinked lines. Flex hoses are new from Stoptech. The pedal returns after cooling-- about 2 laps worth when using no brakes, based on actual race experience. I happily included myself and my braking as a possible source of the problem, but traqmate data shows that I'm not overusing the brakes relative to others (at least not excessively so).

 

Let me know what you think.

 

Thanks

-Randy

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kbrew8991

something that popped into my mind - how shielded are the lines, master, and reservoir from sources of heat (like say a header, uppipe or downpipe)?

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clydesdale
something that popped into my mind - how shielded are the lines, master, and reservoir from sources of heat (like say a header, uppipe or downpipe)?

 

Pretty well shielded. The master cylinder and ABS systems are both at opposite corners away from the turbo. I have seen aftermarket systems that routed past the turbo with problems so you make a good point, but this is fully OEM. I will review the OEM brake line routing regardless. Thanks.

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ILIKETODRIVE

Just to toss this in there due to the fact that I went through both of these with my car.

 

1) A barely barely BARELY internally leaking master cylinder

2) A slightly slightly SLIGHTLY leaking brake caliper from the piston seal

 

I did about 2 weekends before replacing MC and an entire year before changing calipers (found it before then but decided to be cheap and lazy lol) Neither screwed with pedal travel while doing TT (3-4 laps/6-8 minutes) BUT during a race (25+ minutes) my pedal travel would be really REALLY long. Double of normal/halfway to the floor. I noticed that the pedal would sink while holding the brakes with the vehicle off BUT it would take a long time holding them and it would barely sink (MC leak). I then later noticed a small amount of moisture inside my RF wheel and on my right outer tie rod, which led to the discovery of the caliper leak.

 

Just my experience. In no way shape or form did I boil the fluid.

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clydesdale

I decided that replacing the MC was the first obvious thing to do, I'll see what happens after that.

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erioshi

Late to the party, but when you upgraded to the new calipers did you re-size your master cylinder(s) to match? If the new calipers have larger or more pistons (more caliper piston surface area), then you have increased the amount of fluid transfer required when the brake pedal is pressed to achieve the same braking force. That means your pedal would need to travel farther down. This seems unlikely, but if the required increase in pedal travel is large enough, then you may be having trouble putting enough pressure into the calipers to use them effectively when the car is at speed.

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clydesdale
Late to the party, but when you upgraded to the new calipers did you re-size your master cylinder(s) to match? If the new calipers have larger or more pistons (more caliper piston surface area), then you have increased the amount of fluid transfer required when the brake pedal is pressed to achieve the same braking force. That means your pedal would need to travel farther down. This seems unlikely, but if the required increase in pedal travel is large enough, then you may be having trouble putting enough pressure into the calipers to use them effectively when the car is at speed.

 

Thanks for the reply. The new calipers are essentially the same as the old ones as far as piston size and count front and rear. The rotors are fractionally larger but the overall vehicle bias is probably the same as well. All this assumes that Stoptech sent me what I ordered since I didn't actually check piston sizes.

 

As an update to my original post, I'm putting in a new M/C as well as replacing the booster and the pedal connecting rod. The booster will also be configured to allow me to cap the vacuum and run without brake boosting. I plan to maximize the allowable free play between the rod and M/C plunger too.

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DanSTi

Do you have a picture of your ducts? The design of the ducting makes a big difference, and the STi really needs good ducting up front for the brakes to last.

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clydesdale

I don't have a photo handy of the duct itself, but it runs straight from the intake to the hub with a custom flange pointed at the backside of the rotor hat.

 

Here is the intake:

 

duct_sm.jpg

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bob-e

Maybe try RBF 660? A little higher bp supposedly. Titanium brake pad backers? Search for DUNK's posts on NASIOC. He did double ducting and water misters.

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MN-STI

Randy, did you find a resolution to this issue? We've been experiencing the exact same thing while running the stock calipers, 2 piece rotors, DTC-60 pads, and 3" brake ducting.

 

Also, can you say how running DTC-60 front and DTC-70 rear performed versus just the DTC-60 all around?

 

Thanks,

Spencer

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Matt93SE

Edit.. Dammit, I just posted and then realized the last post was 9 months ago.. well I've dun said it already so I hope it helps someone else...

 

[armchair interweb aero expert]

something to keep in mind.. Those corners on the front bumper are *typically* lower pressure zones on the front end than closer in toward the center of the bumper.

 

Take a look at this CFD shot I found while googling..

http://mathieuhorsky.files.wordpress.com/2013/06/wrc_01.jpg

source: http://mathieuhorsky.wordpress.com/tag/subaru-impreza-wrc/

 

Now this is 30 sec of interweb searching, but that CFD shot tells me those brake ducts aren't doing a whole heckuvalot because there's little pressure forcing air through them, even though it's at the front of the car.

 

Compare the location of those ducts to the Aussie V8 cars, which have a roughly similar body shape (if you're measuring with hand grenade/horseshoe units).

http://images.watoday.com.au/2009/11/13/858375/article-V82-420x0.jpg

 

Roughly similar on the NASCAR bodies: http://www.racesonoma.com/images/171236402-lg.jpg

 

Notice how the brake ducts are much closer to the center of the opening, which is a higher pressure area to force more air through the ducts.

 

In lieu of moving your brake ducts and cutting up the bumper, you might consider:

1. Fab some covers for them where the opening is only at the inside edge- roughly similar to the early model STi covers that had the small slits on the inside edge of the covers- just make the holes bigger.

http://extremedimensions.com/images/T/04_imprezastilookfront.jpg

 

2. Buy some brake duct fans to force more air through the ducts. You can buy them used from NASCAR teams at lots of places online for fractions of new cost.

 

Just some ideas- hope that helps. I can't imagine any other reason you would be having fluid problems with that setup unless you were either dragging your brakes or weren't actually getting air to the brakes.

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clydesdale

Either way, I appreciate the follow-up.

 

Replacing the master cylinder was the fix that worked. The disassembled M/C showed no sign of failure but it must have been unable to fully relieve line pressure. Excess pressure was causing the brakes to drag leading to all sorts of problems as you can imagine.

 

As a side note and for posterity, ABS is hugely important for the STI. My impression is that the car has a fairly significant rear static brake bias which gets masked/corrected by the ABS EBD system. I was not able to brake nearly as effectively without ABS because of the rear lock-up, no selection of mixed compounds were effective either. Unfortunately I've also had ABS failures on occasion so having a working ABS warning light is critical. Keep in mind the brake system did not have any prop or bias control; with those added in a non-ABS system you'd be able to correct many of the problems I had.

 

Moral of the story: fix the $150 M/C before fixing the $3500 brake calipers.

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Matt93SE

Glad you found the problem. I wouldn't expect a master cylinder to cause those problems either, but obviously they do. typically when my brakes are dragging after a race, a caliper rebuild would do the trick. I've never had overheating problems during a race though.. then again, I'm running Superlites on a 2600lb car with 150hp. I use $5/qt brake fluid and bleed it every other pad change..

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MikeAtl

Glad you solved the problem, but since the thread has been resurrected, I thought I'd toss this out for others with similar problem, but a completely different solution.

 

A buddy had pedal to the floor issues at Road Atlanta that caused a thankfully minor off in turn 1. No fluid loss and pumping returned the pedal, mostly. The symptoms were similar to what the OP stated, but it turned out to be pad knockback, caused by worn front wheel bearings. Nissan 350Z with the Akebono BBK. New front wheel bearings solved it.

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Calif_Kid
A buddy had pedal to the floor issues at Road Atlanta that caused a thankfully minor off in turn 1. No fluid loss and pumping returned the pedal, mostly. The symptoms were similar to what the OP stated, but it turned out to be pad knockback, caused by worn front wheel bearings. Nissan 350Z with the Akebono BBK. New front wheel bearings solved it.

 

I believe that you can still have pad knockback even with new/good front wheel bearings. Thread http://honda-tech.com/showthread.php?t=1058880 has some info on pad knockback, and post #25 is from a friend of mine who experienced pad knockback at Sonoma/Infineon at 2:35 into the video. The suggestion is that after running over good size curbs, to give the brake pedal a few really light taps with the left foot. - Jim

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MikeAtl

Man, just when you think you solve something...Thanks for the info.

 

I have gotten into the habit of a quick left foot brake check before heavy braking.

 

Mike

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Tjyak50

Ive heard tell of this "knockback" and still cant visualize exactly what is happening.

Someone please re-explain for me?

Tom

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