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Odd Brake Behavior...

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Hello all

Everyone has been so helpful in sorting out some issues with my 13 GT so I thought I'd post a problem I'm trying to help a friend sort out.

My friend bought a 2011 GT race car originally built by Phoenix. The car is set up very nicely with Penske shocks and the whole works. He ran it a dozen times this summer in HPDE but has hopes of doing an AER event this coming summer.

2b90de9b744a066a74d8205368bbb0b0.jpg

The car has the stock four piston brembos on the front and single pistons in the rear. He replaced the front calipers in the fall with remanufactured ones and runs SRF and 3" brake ducts. The first event with the new calipers was fine and three days at WGI so the certainly got a workout and performed perfect. The next even was a day at NJMP thunderbolt. The brakes would be great for part of a session then all the sudden would lose pedal. He would have to pump them back up as if there was air in the system. We tried bleeding the brake and didn't get any air but a lot of black crud from the calipers. The reservoir looked great in color. The car still has the traction control from what I understand and he turns it off everyone because the car acts up if it's on.

I feel that maybe something funny is happening with the ABS? He said the previous owner was trying to get the ABS to work properly but had trouble. While driving he's not aware that the ABS has ever kicked on so we believe it is inactive.

What are your thoughts?

Thank you!

Jake
 

TMSBOSS

Spending my pension on car parts and track fees.
6,860
4,124
Exp. Type
HPDE
Exp. Level
5-10 Years
Illinois
Different track with different heat issues. I has similar results after running at Blackhawk and VIR no issues. Then Road America caused the exact same symptoms. No air in fluid but had to pump before pedal was high/solid on track.

Could be a build up of brake materials on the rotor causing push back of the pistons. A warped or worn rotor can display the same problem.

Check for run out, hot spots and worpage on rotors
 
It should be pretty easy to check the ABS, just stomp on the brakes and if they lock up and hold it's not working. If it's not working that's where I'd start as you suggested.

@blacksheep-1
 
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TMSBOSS said:
Different track with different heat issues. I has similar results after running at Blackhawk and VIR no issues. Then Road America caused the exact same symptoms. No air in fluid but had to pump before pedal was high/solid on track.

Could be a build up of brake materials on the rotor causing push back of the pistons. A warped or worn rotor can display the same problem.

Check for run out, hot spots and worpage on rotors

Watkins is way harder on brakes than thunderbolt so I feel something happened or is wrong but I understand your point. I'll have to explore the rotors more. Didn't really think about a warped rotor causing that issue.

What ended up being the problem with your car?

Thanks
 
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VoodooBOSS said:
It should be pretty easy to check the ABS, just stomp on the brakes and if they lock up and hold it's not working. If it's not working that's where I'd start as you suggested.

@blacksheep-1

We took the motor out for a rebuild so that will have to wait. He told me that in all the time running it he never felt the ABS kick in but I know he never tried like you said. I feel like one time I read something about a deal or something in the ABS system that can go bad it can't for the life of me remember. Maybe it had something to do with the booster?

Jake
 

TMSBOSS

Spending my pension on car parts and track fees.
6,860
4,124
Exp. Type
HPDE
Exp. Level
5-10 Years
Illinois
jpt3 said:
Watkins is way harder on brakes than thunderbolt so I feel something happened or is wrong but I understand your point. I'll have to explore the rotors more. Didn't really think about a warped rotor causing that issue.

What ended up being the problem with your car?

Thanks

Rotors being .010 to .018 out of true. None were straight.
 
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Based on your description of loosing the pedal, I assume the pedal went to the floor as opposed to becoming hard.

In addition to rotor run out, a bad front wheel bearing can also cause excessive pad knock back. But, both of these would present themselves all session and not simply at the end.

The black crud makes me think the system needs a complete flush. Who rebuilt the calipers? He might disassemble the front calipers and inspect the piston seals for degradation. Also, inspect the brake lines, especially if there are any rubber lines still on the car.
 
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2012-Boss said:
Based on your description of loosing the pedal, I assume the pedal went to the floor as opposed to becoming hard.

In addition to rotor run out, a bad front wheel bearing can also cause excessive pad knock back. But, both of these would present themselves all session and not simply at the end.

The black crud makes me think the system needs a complete flush. Who rebuilt the calipers? He might disassemble the front calipers and inspect the piston seals for degradation. Also, inspect the brake lines, especially if there are any rubber lines still on the car.

The calipers were remanufactured units he bought so whoever does the remanufacturing. He did not do it. I guess if there was excessive knock back that could make sense. In terms of a flush wouldn't you expect the crud to get into the reservoir as well because it always looked clean. It makes sense the seals may not be holding up except he wasn't losing fluid.

I love figuring this stuff out.

Thanks

Jake
 
What you're describing sounds like "knock back", which can happen when rotors "clock" ever so slightly due to the axial loads that occur during hard cornering, enough to push one or several pistons slightly back into the bores. The result is when you get on the brakes in the next corner you get the excessive pedal travel you are describing.

Here's a good video in which a Stoptech engineer explains it, along with other good to know info on performance braking systems. Knock back is fairly common with one piece or non-floating two piece rotors. Going to floating two piece rotors helps to mitigate this phenomenon, a big reason (along with allowing for thermal expansion) that most race cars use them.

https://youtu.be/vdX3WdZLXnc
 
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That's a great video. I guess we will start by evaluating the rotors and go from there. The k member is out right now due to the motor coming out so I guess will take the rotors to a shop and see if they need turned. That's probably the place to start. If they are rue then it's time to look at the calipers. To me it would be odd if it was all the suddenly happening with calipers when it didn't before. The rotors naturally could have gotten warped but the pedal didn't pulsate. I'll keep you guys updated.

Thanks


Sent from my iPhone using Tapatalk
 
jpt3 said:
That's a great video. I guess we will start by evaluating the rotors and go from there. The k member is out right now due to the motor coming out so I guess will take the rotors to a shop and see if they need turned. That's probably the place to start. If they are rue then it's time to look at the calipers. To me it would be odd if it was all the suddenly happening with calipers when it didn't before. The rotors naturally could have gotten warped but the pedal didn't pulsate. I'll keep you guys updated.

Thanks


Sent from my iPhone using Tapatalk
Just check the rotors with calipers. Would not waste my money on turning them unless you have something special rotor wise.
especially on a race machine...
Brakes are the most important item on the car. Never liked rotors that were turned down. Never had any luck with them.

Put on a good set of front centric blanks for around 210.00. You may even find them cheaper.

Keep us posted on your findings.
 
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superhero said:
Just check the rotors with calipers. Would not waste my money on turning them unless you have something special rotor wise.
especially on a race machine...
Brakes are the most important item on the car. Never liked rotors that were turned down. Never had any luck with them.

Put on a good set of front centric blanks for around 210.00. You may even find them cheaper.

Keep us posted on your findings.


How do you check run off with just a caliper? Are you saying it will have varying thickness?
 

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
2,009
438
Southeast
You will need a dial indicator with mounting base to check the Run-Out of your rotors. And No, your not checking for a thickness variance, your checking for warpage of your rotors. The dial indicator point should be within 1.0" of the outer diameter of the rotor.

Significant rotor warpage will also give you brake pedal isolation or shutter when you apply hard braking. Remove the caliper to measure for out-of-round rotors.

You are correct, as a caliper is used to measure rotor thickness. Compare this thickness reading with the Minimum Rotor Thickness that is embossed on your rotor hub.

Good luck and keep us posted.
302 Hi Pro
 

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