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Old 10-31-2020, 09:22 PM   #1
ZL1GMA
 
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Brake Pedal Soft/Long Travel after Track - Pad Taper or Air?

Recently completed my first track day at Buttonwillow in my '19 ZL1 A10. Completely stock.

I flushed the brake fluid with Brembo HTC 64t (DOT 4) before the track day and the pedal felt pretty good.

The brakes felt a little soft after the track day. They still stop strong but the top couple inches of travel feels softer than before when just depressing the pedal while the car is idling.

I tried bleeding the brakes for ~6 hours today. Used the two-person method, also tried with a Motive power bleeder, and used GM GDS2 scan tool clone for automated brake bleed to make sure I got the ABS module. Did (2) automated bleeds.

The pedal feels a bit better but not as strong as I think it should. Could the long travel be caused by pad taper? I did not pull the pads but wanted to check if anyone else has this issue.

If I find they are tapered, should I swap from side to side to correct?

Anything else I should try?

Thanks
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Old 10-31-2020, 09:25 PM   #2
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Having same problem as well after a recent 100 degree buttonwillow event

Curious for any input here
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Old 10-31-2020, 10:24 PM   #3
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How do the pads look? Sounds like air from common boil-over, but you said you bled to take care of that.

I think after bleeding any change in thickness should be offset and hard pedal resumed, unless they're really worn. Maybe check pads before bleeding, pads can be removed/replaced fast :-)
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Old 10-31-2020, 11:54 PM   #4
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I don't think it's possible that I boiled the brembo fluid. I have boiled brake fluid before and the pedal has 0 pressure when it boils. I made sure to let the brakes cool off before parking in the pits so I don't think it boiled while parked.

I will definitely pull a pad to check taper and consider getting new ones to see if that helps. I wanted to see if anyone has had an issue with pad taper causing a longer/soft brake pedal first.

I could also keep trying to bleed it, but would probably try vacuum or gravity since I already tried the motive pressure bleeder for several hours.
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Old 11-01-2020, 12:49 AM   #5
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Not sure about the Gen 6 cars but on my Z/28 it has a non-traditional bleed pattern of RR LF LR then RF. Seems like you've done everything else.
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Old 11-01-2020, 12:55 AM   #6
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Quote:
Originally Posted by ariZona28 View Post
Not sure about the Gen 6 cars but on my Z/28 it has a non-traditional bleed pattern of RR LF LR then RF. Seems like you've done everything else.
This is the order I did for the brake flush before the track day, and the power bleeds I did after to try to solve the issue.

For some reason, the GM GDS2 automated bleed procedure has an order of LR, LF, RF, RR, and the abs module only activated audibly on the RF and RR calipers... So I followed that order during the automated procedure.

After each automated procedure I did the usual RR, LF, LR, RF order with the motive bleeder & some extra pumps of the pedal.
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Old 11-01-2020, 07:05 AM   #7
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pedal travel is longer after pad wear (especially the rear pads) as that is the initial portion of the brake pedal travel. Double check for air pockets and bleed both caliper bleeders inner then outer in the sequence mentioned above.
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Old 11-01-2020, 07:06 AM   #8
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Quote:
Originally Posted by ariZona28 View Post
Not sure about the Gen 6 cars but on my Z/28 it has a non-traditional bleed pattern of RR LF LR then RF. Seems like you've done everything else.
yes, 6th gen uses this pattern as well with the "crisscross" circuit
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Old 11-01-2020, 09:13 AM   #9
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Use a dead blow/rubber mallet on the calipers when you have the bleeder open. Sometimes you can vibrate some captured air out of the caliper. No need to hit the caliper hard and stay away from the pad pins and bottom crossover tube.
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Old 11-01-2020, 09:35 AM   #10
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Quote:
Originally Posted by cdb95z28 View Post
Use a dead blow/rubber mallet on the calipers when you have the bleeder open. Sometimes you can vibrate some captured air out of the caliper. No need to hit the caliper hard and stay away from the pad pins and bottom crossover tube.
Tried tapping the calipers with the mallet while bleeding yesterday. I saw something on C8 forum that was interesting, the official brake bleed procedure involves retracting the pistons. Might have to try this if I can't solve it. Already tried cycling the park brake while bleeding but it didn't seem to help much.

Post #7 in this thread: https://www.corvetteforum.com/forums...iscussion.html

GDS2 also has a way of retracting the rear caliper pistons through the computer, might try that to see if it helps release some air.

Just curious how a worn pad would increase pedal travel while just stopped
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Old 11-01-2020, 01:30 PM   #11
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I am a betting a powdered doughnut that it is pad taper. I have tracked cars for 20 years probably 15 different cars and nearly every stock caliper eventually caused some pad taper, some were extreme (looking at you C5 Z06), but only boiled fluid once. With the size of these brakes and that fancy HTC fluid I really doubt you boiled it, especially with stock pads.
Pad taper gives a soft pedal because the pad wears unevenly sometimes both laterally (front to back) as well as the more common top to bottom wear (why differential bore calipers were invented). The taper causes a soft pedal as the uneven pads now takes a little more travel to get the whole pad pushed squarely against the rotor again when you press the pedal. In my experience they typically feel almost normal again after a few weeks on the street if they aren't tapered to bad.
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Old 11-01-2020, 07:14 PM   #12
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Thanks for the insight. It has only been a couple weeks since the track day and I've only driven the car a few times.

I will order new pads just to see if that fixes it. Would be a good data point & I can keep them for later too.

Do track pads have less taper issues?
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Old 11-03-2020, 01:18 AM   #13
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Quote:
Originally Posted by ZL1GMA View Post
Tried tapping the calipers with the mallet while bleeding yesterday. I saw something on C8 forum that was interesting, the official brake bleed procedure involves retracting the pistons. Might have to try this if I can't solve it. Already tried cycling the park brake while bleeding but it didn't seem to help much.

Post #7 in this thread: https://www.corvetteforum.com/forums...iscussion.html

GDS2 also has a way of retracting the rear caliper pistons through the computer, might try that to see if it helps release some air.

Just curious how a worn pad would increase pedal travel while just stopped
I’m really curious how it does that.
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Old 11-06-2020, 11:18 PM   #14
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Update: I ordered new pads and installed them today.

The pedal is back to feeling great, instant response and no long pedal.

The old front pads are about 50% worn. They have about 1mm of taper, and the rears have about 0.3 -0.4mm of taper.

Not sure if I'm just sensitive to this kind of thing, but the new pads made a huge difference to me.
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