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No brake pressure after track sessions.

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#1
I have an issue with my Fiesta where while on track the pressure the pedal pushes back at my foot reduces, and that correlates to the calipers not applying pressure to the pads. Basically my brakes start getting really really vague. I have used Carol SRF for the last two years, and this isn't the same feel as boiled fluid. The pedal pressure/effort never comes back. If I pump the brake pedal with the engine off the pedal gets hard, but as soon as the engine turns on the pedal pressure disappears and I can push the brake pedal pretty much to the floor.

My friend who also tracks a Fiesta has this same issue, and between the two of us we've been fighting this issue for almost a year. I have swapped my brake lines, master cylinder, and booster. The booster fixed the issue until my next dry track day (in rain track sessions the brakes behaved normally, but weren't pushed nearly as hard).

My friend has replaced his master cylinder, brake booster, and ABS unit. The ABS unit fixed the issue for a week on the street (he says he tried to make the pedal go soft on the street, but we both have a limit to what we'll pull on the street). The first session last Saturday he said the brakes were fixed and confidence-inspiring. By the second session the problem had returned, and at the end of the day they were back to vague and he could push the pedal back to the floor. Sunday morning they had returned a bit, but by the end of the day he could again push the pedal to the floor.

Here's a weird twist: last week I had the race shop I use replace the front pads because I have had issues where the brakes lock up after pad replacement or brake bleeds. The tech noticed that my pedal wasn't hitting the brake switches in the right place, and adjusted where the pedal hits the switches. My pedal was vague before this, and back to normal after. I had a wrench light and code P0504 for the brake switch, though. I was happy and the brake lights worked normally (I passed state safety inspection the next day). In my first session Saturday, the brakes behaved normally until I cut a corner through the dirt, which was a bouncy excursion. Immediately the wrench light turned off and at the next heavy brake zone my brakes were almost nothing. They came back a bit, but only back to the vagueness from before the pad swap. Sunday I tried unplugging the cruise control switch, and the brake code came back but the pedal pressure didn't.

So here's my question: has anyone encountered this sort of thing? Or do they know if our cars have some sort of "ice mode" where the car's computer freaks out at repeated hard braking like FRS/BRZs apparently do?

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#2
I have been experiencing similar issues and it is VERY frustrating. My setup is Carbotech XP-8's with Centric rotors and Mountune SS lines at all four corners. Fluid is Castrol SRF. The front rotors are also ducted. I was hoping the SS lines would help with pedal feedback and it does seem to help with modulation under threshold braking on the street but at the track it didn't make much difference, or the root cause is overshadowing any improvement from the lines. Initial bite is terrible and I have to press the pedal really far to get any friction.

Your comments about the ABS/brake switch is interesting to me because in addition to the long and inconsistent pedal travel it does also seem like ABS has been engaging early, which doesn't make sense at all. I've always thought that ABS activation is based purely on the wheel speed sensors but it really seems like there's something more at play because when I'm at the track it definitely engages earlier than an equivalent hard stop on the street. I wonder if pulling the ABS fuse or disconnecting the stability control/ABS module would help to determine if it is a factor? I wouldn't suggest keeping it bypassed but if we can remove it from the equation temporarily and prove that there is a noticeable change then that could be productive.

I really don't want to have to spring for a race-grade BBK but I'm starting to feel like I've exhausted all other options at this point. This is just destroying my trackdays. It's frustrating to see and hear about other folks who apparently have no issues at the track but I guess it boils down to some of us being much more demanding on brakes as our pace increases.
 
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Thread Starter #3
That sounds a lot like my brake setup, except I stepped the G-LOC/Carbotech pads up to 12s in the front and 10s in the rear. I'm glad the local guy and I aren't the only ones dealing with this.

Right now, to mitigate this a little, I'm bleeding off vacuum with a boost controller hooked into the vacuum line going into the brake booster. It helps with feel, but if you push deep enough into the pedal (or vent too much vacuum) you just can't push the brake pedal any harder. This isn't a solution and just masks the problem (and can fuck up traction control), but it gets me around town for now.

What I'd really like to do is to get some "jumper" lines plumbed that bypass the ABS block entirely and see if that brings back the pedal behavior/pressure I expect. My current theory is that something has gone wrong in the computer module for traction control/torque vectoring and a valve in the ABS block is stuck open or somehow damaged. I tried pulling the ABS fuse under the hood and it didn't fix the pedal-to-the-floor problem when I turned the engine on.

Also, this isn't entirely related, but when your brakes were behaving normally, did you notice a difference in where the brake pedal resisted your foot depending on what the car was doing? I noticed that in the beginning every time I tapped the brakes while torque vectoring was engaged I had a really hard pedal at the top of the pedal travel, but if I was just braking in a straight line I had a normal pedal. I'm pretty sure that was because the ABS module was doing wizardry to try and turn the car.
 
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#4
Maybe things are flexing under the heat and strain? These are cast parts that can get bent easy. At some point the brakes are going to stop pushing against the rotor and instead start trying to bend the caliper brackets and slide pins.
 
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Thread Starter #5
I replaced the front calipers last August, after two years of track days (20-24 weekend-long events). And then rebuilt the front calipers a month ago. Pad wear was even left/right and inside/outside when I swapped pads last Friday, so I'm assuming the calipers aren't physically damaged.

Right now the shop that has been keeping me on track is trying to reinforce the brake pedal. I have bent the brake switch bracket bleeding the brakes a couple of times and locked the brakes, which is why I'm thinking this is an electronics/ABS computer problem and not a hardware problem.

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#6
Also, this isn't entirely related, but when your brakes were behaving normally, did you notice a difference in where the brake pedal resisted your foot depending on what the car was doing? I noticed that in the beginning every time I tapped the brakes while torque vectoring was engaged I had a really hard pedal at the top of the pedal travel, but if I was just braking in a straight line I had a normal pedal. I'm pretty sure that was because the ABS module was doing wizardry to try and turn the car.
I can’t comment on applying the brakes during TVC activation but I have noticed some differences in pedal pressure and feedback on different parts of the track. Ie breaking in a straight line vs a little tap during corner entry. I chalked that up to rotor displacement when the wheels are loaded laterally.
 
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Thread Starter #7
Update time:

I drove my friend's track Fiesta ST that had brake problems, his entirely stock daily Fiesta ST, and my own Fiesta ST. All back-to-back (we were transiting 3 cars and two drivers between the race shop and our respective apartments).

I have replaced the pedal switches. He has replaced his ABS module. The stock car was a control for what our brakes should feel like.

All three cars have pretty much the same brake feel, within small tolerances for different brake fluids and pads. Bleeding the brakes would probably get everything even closer.

New switches have solved my pedal problems. I'm at the track this weekend with the reinforced brake pedal so hopefully it'll stay that way. The new ABS module solved my friend's problem, even if it wasn't promising at first. He hasn't had any problems since the first track day.

So, @SonicDeathmonkey, try replacing your brake switches if you want your pedal to come back for at least the street (I'll update if it holds up to my track abuse). It's under $20 to get both Motorcraft switches from Rock Auto.


EDIT: I also did a pad change a few weeks ago. The pad wear was almost perfectly even. Better than I've seen in a few years, so I'm pretty confident that nothing near or on the caliper is bending.

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Thread Starter #9
Update on the brake "fix": I was at MSR Houston this weekend and I had a reinforced brake pedal assembly installed last week.
Here's some of what was done to reinforce the pedal:
[EDIT: nevermind see the next post for the images of the reinforced pedal]

The new pedal and switches brought my brakes back to great on the track. The pedal was consistent and the brakes were strong. On the street after the Saturday they pedal was a bit soft, but it felt like boiled fluid (which I've done a lot), and on Sunday the pedal was strong and immediate after two warm-up laps. I want to try bleeding my brakes when I get the chance, and I expect the pedal will come back once I find someone to help me.

Have you checked the rear caliper, and how the pad (namely the nub) is sitting in grove on the piston?
I actually had the wheels off today and I checked the rear calipers: they're installed correctly.
 
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#11
So you think the pedal assembly could be bending under the strain? It wouldnt suprise me much.
 

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