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Brake pedal firm at first but goes soft if holding pressure

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Was wondering if anyone experienced this. Mostly happens on track, I brake hard at the end of a straight before a hairpin (think hairpin at bedford at the end of the second straight, after the 1km straight - which is a very hard braking zone, from 100+mph to 35-ish, ), The brake pedal is quite firm for the first 1-2 seconds, but then the pedal goes down slowly, almost like the brakepads became wooly and compressed 50% or something loosing pressure somewhere.

It does not go all the way to the floor, but significant enough to feel it softening.

It does not seem to affect the braking itself, can still control the slow down, and then trail brake, and it is fine for the next corner.

I also never felt unsafe, since the actual braking was not affected, but weird feeling to have the pedal go soft in the middle like that when held under pressure for a bit longer.

Again this only happens when I'm really standing on the brakes, 100% pressure, after at least 1 or 2 seconds, not right away.

I've got BBK in fornt (Stoptech) and stock calipers in the rear. Stainles lines in front. rubber ones in the rear.

I had the pads changed not too long ago and brakes bled. Brake fluid is Millers racing fluid dot 4.

Had this happen both with EBC Yellows and Stoptech Sport pads

Any ideas ?

 

 

 

 

 

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Getting some brake fade there mate, i get that on standard calipers and pfc z pads. Time for some proper pads such as pfc 08s

 

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I have brembos on mine, using performance friction z rate atm. Will switch to ds2500s shortly - only due to personal preference. Never had fade on either across different cars on trackdays.

How recently was the fluid changed? Within the last year?

The fact you've had it across two different pad types makes me curious.

Personally after few trackdays I bleed off some fluid from the calipers to keep what's in there fresh. I would be tempted to bleed some off for two reasons, in case of any trapped air and to check its colour. If there is no air and the fluid looks fresh you know the issue lies elsewhere.

After that check you disk wear, although I assume your bbk is fairly new?

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Ebc yellowstuff/pfc z/ds2500/stoptech sport are all trying to do the job of road and track, they are limited on track, if your braking hard and often they will fade. Maybe not fully to the point of no braking but they will fade

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Ebc yellowstuff/pfc z/ds2500/stoptech sport are all trying to do the job of road and track, they are limited on track, if your braking hard and often they will fade. Maybe not fully to the point of no braking but they will fade

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Depends how you brake and how long for. At snetertton, 130 mph down to 40-50 never any issues with fade. I had ds2500s on the rx7 and no issues on that either. I am on road tyres and I run 20min sessions on track.

Fundamentally these cars are not heavy so dont need much to slow them down.

If you're on semis the pads will be working harder however

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Stopetch sport pads in the rear, with stock caliper, OEM rubber lines, and Stoptech uprated disks.

BBK installed and liquid flush in December with Stoptechs.

Had a flush and EBC yellow pads installed in April.

Had pads changed again to new Stoptechs in June, nu flush, still doing it.

I've got a Master Cylinder  brace also, and brake ducts.

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Point is OP is experiencing brake fade on pads that are limited in performance

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Point is braking technique affects the amount of heat generated, point is maintenance affects braking performance as well. If you go for a full blown race pad you compromise on the road - which is fine if the OP wants that.

There are many people of the forum using fast road style pads with no issue on track and in the sprint series.

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So I take it you guys think it is most likely  brake fade from pads overheating, rather than a bad MC or anything else ?

Would the pedal go down if the brake pad was overheating?

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So I take it you guys think it is most likely  brake fade from pads overheating, rather than a bad MC or anything else ?
Bad mc would give you an issue all the time in my view, not just when your on track.

Do you drive the car solely on track or do you use it as a road car as well?

How long are you running out on track? 20, 30 mins etc?

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 I don't feel a drop in braking power but rather a softening of the pedal, that feels almost hydraulic (like when you release the valve of a hydraulic jack stand). I might have fluid fade rather than pad fade.

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How many track days are you doing a year?

Did you flush the fluid?

Are you using millers dot 4 300?

Without being rude, how are you braking? It's better to brake harder for a shorter period of time versus less pressure for a longer amount of time. I have seen many people on track days riding the brakes then wondering why they get fade.

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Around 20 track days per year (2/month).

Yes, Millers dot 4 300

I have been told to brake later and harder, and I do tend to stay on the brakes a bit longer than my instructor likes it, so I see your point.

Still, it feels like my brake pedal has 2 modes of operation.

1) If I am light on them they don't fade. Ever.

2) If I am hard on them, like when I do brake later and harder, there is the initial application (I can even stomp on them, they will be firm), and then they compress.

I don't experience any 'fade' per se in braking power. It's just the pedal "compresses" - soft hydraulic feeling of lowering for about half an inch - after one or 2 seconds if I stand on it. Braking power still there.

 

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So I have speed bleeders on my brembos, every other trackday I bleed some off to keep it fresh as the fluid just sits in a hot cailper. Takes 30 mins tops. I'd be tempted to try that and see if that helps, also you're banging in the trackdays (bravo!). The fact you're getting pedal sag makes me think the fluid is giving up. Once you boil fluid it's more prone to sagging later. I just use cheap dot 5.1 as by definition it has a better boiling point that dot 4 (not better than the racing stuff mind) with the view of refreshing it by bleeding as I say. As you may already know, brake fluid is hydrosopic so it ages over time, the harder you are on it the shorter it lasts.

 

Another thing to try is adding some ducting to your brakes if you have not got this already and for the cost of it I would get rid of those rubber lines!

 

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Thanks guys, I really appreciate it.

I'll look into some  Ti shims and better quality pads (Pagid RS29 )and see what happens.

@Sentinal187   I will try refreshing the fluid by bleeding it and rear braided lines.

Cheers!

 

 

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If you pushed the pedal further does the Abs engage? I do think it's brake fade your getting also but just gauging how far your pushing the brakes. 

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28 minutes ago, Luke said:

If you pushed the pedal further does the Abs engage? I do think it's brake fade your getting also but just gauging how far your pushing the brakes. 

I don't think ABS  was engaging, not in the usual way at least., the pedal just sags for half an inch but pressure is still controllable, I don't feel loss of braking power. Everything feels normal except for the bit of sagging in the middle. Again this is when I'm standing on the pedal, braking late and hard after a long straight, and I am still  going in a straight line, slowing down from 100+ to 35.  It feels like some pressure is released somewhere (feeling is hydraulic/progressive, not abrupt), but no vibration or rock-hard pedal. 

While my ABS was still ON, I was in Pedal Dance mode otherwise.

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On 7/22/2019 at 9:26 AM, james_ly said:

Respect! 👍

Been doing it for 2 years now, in conjunction with about 10 Car Limits handling days per year.

I wish it was all fun, but buying new tyres (PS4) every 4 month, having full service (engine, tranny, diff, brake flush)  and new front pads every 2-3 month, gets a little tiring sometimes.

Not counting the cost of track days and full day instruction (which is worth every penny imho).

And the more you learn, the more you realize how much more there is to learn (and how much you forgotten since the last session). Can be a bit discouraging sometimes.

But the highs are pretty high too when things come out right. :)

 

 

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