Brake pedal pulsing

[Chris McClendon]

I've searched the forums but have not found anything similar to this, so thanks in advance for any advice. I've been doing track days for a couple of years now, mostly with NASA, and have done 3 events in HPDE 2 this year. Road Atlanta is the main track I go to, and I feel comfortable with the line, braking zones, turn in, etc. I have a 2006 Mustang GT with the Brembo 4 piston brake kit, stainless lines, stock rear brakes, and dot 4 fluid. I'm running Centric rotors and G-loc R-12 pads front and R-10 rear. I've started to brake a lot later and harder into the brake zones in turn 1, turn 6, and turn 10a. I'm well into the the anti-loc engagement. The brakes hold up well, and I don't get any fad during the session. What I have started experiencing is the brake pedal "kicking back" or pulsing during release, or with lighter pressure (say a 3 or 4 instead of a 10). It doesn't happen all the time, maybe once or twice per session. It has not affected the braking performance, but wondering about the function of the anti-loc system, or the sharing the master cylinder with the clutch. I have experienced the pulsing while releasing the clutch after down shifting, while holding light brake pressure before release.

Wondering if others have experienced this and what it might be related to? Thanks for any insight.

 

[67GTA]

Check your front wheel bearings for play and lateral runout. You didn’t mention brake ducts for cooling - if you’re not running ducts, I would guess your bearing hubs are taking the brunt of your increased braking.

 

[Chris McClendon]

Check your front wheel bearings for play and lateral runout. You didn’t mention brake ducts for cooling - if you’re not running ducts, I would guess your bearing hubs are taking the brunt of your increased braking.

Yes, I am running brake ducts on the front. The wheel hubs are fine, no movement when checked. I don't get vibration through the suspension, just the pedal pushing back under my foot.

 

[Dave_W]

Those are the 3 heaviest braking zones. Is it only on those turns where you're getting the vibration? Have you looked at the rotors for any patches of discoloring (or other issues)? My first thought is inconsistent pad deposits on the rotor. Also try braking sooner and a little less to keep out of the ABS and see if it makes a difference. The ABS coming on means a wheel is starting to lock up and the tire skid. The vibration could also still be ABS coming on if you're turning too much without letting up enough on the brakes.

 

[blacksheep-1]

If it's not the wheel bearings, check the rotor runout, they are prolly getting hot and warping, you might also be experiencing a bit of "knockback" which is when the pads get knocked back from rotor runout down the straights, sometimes you'll have to give them a pump or 2 before getting on them for real. Pretty common at Sebring into turn 1.

 

[Chris McClendon]

Thanks to all for the replies. After a discussion with a friend who has an S197 and a lot of track experience, I think what I'm experiencing is a hard pedal because the brake booster has lost vacuum. The vacuum level, booster, and check valve all check out under normal conditions. It seems as though I'm depleting vacuum in the brake booster under hard braking, and their is not sufficient vacuum to replace it, under certain conditions. Time for a vacuum gauge.

 

[302 Hi Pro]

Chris:

Sorry if I missed this above but when is the last time you serviced/flushed your brake fluid? Also have you separated your Brake & Clutch fluid reservoir?

 

[Chris McClendon]

Chris:

Sorry if I missed this above but when is the last time you serviced/flushed your brake fluid? Also have you separated your Brake & Clutch fluid reservoir?

Thanks Hi Pro - I last flushed/replace brake fluid in March. Have done three track events since then, March, June and August (experienced behavior described at last two events). I have not separated brake and clutch fluid reservoir, and have wondered about that being a potential cause.

 

[302 Hi Pro]

I suspect you used a quality DOT 4 fluid like Motul 6XX or Castrol Racing fluids.

I’ve noted that some Members here recommend a brake flush after every track event which I agree with. Heat at the front brake calipers can be extreme depending upon how aggressive a driver is on braking. (i.e. deep in the braking zones/trail braking, etc.)

Keep us posted on your findings.

 

[302 Hi Pro]

Oh, PS:

Definitely divorce your Brake/Clutch reservoirs as finite Clutch dust makes a mess in your brake fluid reservoir.

Plus I find it makes maintenance much easier as I can now separate their service schedules.

 

[Chris McClendon]

Thanks for the recommendations. And yes, I use Motul 600 brake fluid. Any tips on what to go with for a dedicated reservoir?

 

[302 Hi Pro]

I divorced mine way back in the day when no kits were available. I ended up cobbling together my own using a Chevy Camaro clutch fluid reservoir mounted it to the back side of my strut tower & just connected the Ford OEM clutch fluid line to it. Then to rid my Boss of the brake fluid resivour rubber block-off nipple for the clutch line, I installed an AT reservoir. It’s a very clean install. Paid less than $60 for the 2 reservoirs, as the AT reservoir came slightly used from a Forum Member FS ad.

But today you have you have options on preassembled kits for this purpose. One kit that I like is offered from the Shelby store. Here is a link:

https://www.shelbystore.com/product-p/shel031.htm

A nice looking kit, but early purchasers of this kit had reservoir leaks from the bottom reservoir seal. Call them to verify that is no longer an issue.

I googled this: ‘s197 clutch fluid reservoir kits’ and several popped up at various price points.

Worth the time & money.

 

[Chris McClendon]

I divorced mine way back in the day when no kits were available. I ended up cobbling together my own using a Chevy Camaro clutch fluid reservoir mounted it to the back side of my strut tower & just connected the Ford OEM clutch fluid line to it. Then to rid my Boss of the brake fluid resivour rubber block-off nipple for the clutch line, I installed an AT reservoir. It’s a very clean install. Paid less than $60 for the 2 reservoirs, as the AT reservoir came slightly used from a Forum Member FS ad.

But today you have you have options on preassembled kits for this purpose. One kit that I like is offered from the Shelby store. Here is a link:

https://www.shelbystore.com/product-p/shel031.htm

A nice looking kit, but early purchasers of this kit had reservoir leaks from the bottom reservoir seal. Call them to verify that is no longer an issue.

I googled this: ‘s197 clutch fluid reservoir kits’ and several popped up at various price points.

Worth the time & money.

Thanks for the info!!

 

[blacksheep-1]

while a separate master cylinder will be the eventual way to go, I can't help but think there is something else going on there that is not M/C related

 

[Grant 302]

while a separate master cylinder will be the eventual way to go, I can't help but think there is something else going on there that is not M/C related

OP suggests vacuum loss:

Thanks to all for the replies. After a discussion with a friend who has an S197 and a lot of track experience, I think what I'm experiencing is a hard pedal because the brake booster has lost vacuum. The vacuum level, booster, and check valve all check out under normal conditions. It seems as though I'm depleting vacuum in the brake booster under hard braking, and their is not sufficient vacuum to replace it, under certain conditions. Time for a vacuum gauge.

 

[302 Hi Pro]

while a separate master cylinder will be the eventual way to go, I can't help but think there is something else going on there that is not M/C related

Your absolutely right, and I didn’t mean to infer that it was a cause, only in regard to ease of scheduled maintenance, (see post #10)

Regardless of the reservoir, a fluid change is due after 2 track events. IMO of course.

What are your thoughts on frequency of brake fluid changes after track events?

Thanks,

 

[blacksheep-1]

Originally, he was complaining about "pulsing" not just a hard pedal, lack of vacuum may cause the hard pedal, but not so much the pulsing. Generally, the brake fluid gets changed before any race weekend, and bled at the end of each day. Phoenix uses AP super 600, it's relatively cheap, and it works.

 

[BillyJRacing]

I seem to read a lot of uneven pad deposit symptoms from people running those pads...

 

[blacksheep-1]

………..and Hawk pads

now that it's been mnetioned

 

[Ewheels]

I had a similar brake judder under braking and it was from hot spotted rotors. I bought super cheap Centric C-TEK rotors and they cannot handle the heat. I think possibly your rotors got too hot and are warping, as said above.