So I just got back from my first track day of this coming season this afternoon. I'm becoming more concerned about a problem(?) i first experienced last year and I'm hoping some of you can help me out. Occasionally when I begin to press the brakes going into T1 after a long straight, the brake pedal feels super hard and only compresses about an inch or two. Im still able to stop the car but its not an easy task. With pedal modulation I can get a little bit better feel from the brakes but its still not normal. When it happens, it basically feels as if i don't have power brakes.

It seems to happen when i make a quick transition from throttle to brake. Not unsettlingly quick, but when I ease off the throttle and onto the brake. It also sometimes feels like the abs engages, which is avoidable, again with modulation. If I ease off the throttle and wait a second before applying the brakes it feels pretty normal.

My brake setup: OEM Brembos, SS brake lines, AP Racing Dot 5 fluid, blank rotors, EBC Yellow pads F/R

Ive tested the brake booster as per the manual and the vacuum line is good.

Im rather new to the track, so is this common? Is this related to turbo charged cars? Or what else can I look into to fix this. Thanks!

If my explanation doesn't make sense, this guy seems to have the same problem but no real answer http://www.lotustalk.com/forums/f91/...roblem-100659/

 

It could be the check valve in the brake booster hose is allowing positive pressure into the brake booster or too much pad for the tire causing abs to kick in. Either way I'd lose the 5.1 fluid and go with non silicone DOT-4.

 

I'm usually on 255 nt01s but yesterday I ran 245 AD08s. What is wrong with DOT 5.1? I have no problems changing it.

It seems like the check valve is functioning correctly. (in my driveway). When the engine has been shut off I can depress the clutch with assistance twice before it becomes firm. And when I went to inspect the check valve hose after letting the car sit for about 30min air surged in when I removed the hose from the brake booster. This leads me to believe its working properly, if the check valve is inside the vacuum line.

I'm curious if maybe the valve is behaving differently on track verse idling in the driveway.

To bad I didn't know how all this **** actually worked until I researched it when I got home.

Consensus at the track was that I was overheating the pads and I should try sequentially downshifting. The engine braking helps slow the car down but doesn't affect the pedal feel. Next, im going to look into it being abs like you said

 

+1 for switching brake fluid. Syn or 5.1 fluid is NOT made for heat/moisture resistance that you require on the track. Get a good racing dot3/4 brake fluid like Endless, SRF, AP, there's dozens and dozens that work to choose from which also work on the street but be wise about bleeding often. The one you used is made by AP racing but it's not an actual racing fluid. However the problem you're experiencing at the end of long straights/long boost duration is common among evo's. Basically when you take your foot off the gas and go for the brake too quickly without allowing the positive pressure to be come negative (aka vacuum which makes power brakes work) it's like stepping on a rock because it wasn't given the time to go back to vac. Many people have tried vac reservoirs and all sorts of creative ideas but the problem still exists. So if you're using high boost on a long straight it's driving style that you need to correct - come off the gas 3 or 4 seconds (max) before you know you need to use the brakes and that should fix the issue. Also you could try taking your foot off the gas half way down the straight for a quick bleed and see if your usual braking method works better then too.

 

I had this same kind of issue two years ago.... I replaced everything brake related with no success... then one day I'm under the car and I notice my front right ABS ring has three damaged teeth. Not very severe ... looked like when the axle was out of the car for a clutch job the axle tipped over and impacted a smooth cement surface.

I picked up a new axle from a parts store for $79 and replaced it .... PROBLEM SOLVED!!!

So... take a look at all of your ABS ring gear teeth. You can clearly see them from under the car while free spinning each wheel.

 

Thanks guys, I'm glad I'm not the only one who has experience this. Batting around ideas in my head, I thought i might be able to see some results with the methods you've mentioned. I'm going to take a look at the ABS system as well, hoping everything is fine.

As for the brake fluid, i was planning on switching to ATE super blue so that's what ill do.

I dont mind adjusting my driving technique if that's what it takes. But i didn't want to go that route if I was really neglecting a problem i should resolve.

 

it's because under acceleration (full boost down the straightaway) the brake booster (vacuum operated - the opposite of boost) doesn't build vacuum, there for doesn't power assist braking until you've been off the throttle for a second letting it accumulate some vacuum.

 

^Only true if the check valve in the brake boost hose or the brake booster reservoir is faulty, or you left foot brake more than once while still under boost.

 

Yes, and if this was true you would expect it to happen at every corner. Unfortunately I haven't had the time to look at it this week. Hopefully ill be back this afternoon early enough. I wonder how it would behave in autox situations

 

What tire are you using Oki?

 

255 NT01s typically. Last weekend I was on 245 AD08s

 

What Jeff said or more tire less pad.

 

Yea im starting to think it is an ABS problem (which i haven't looked at yet) or the pads. If it boils down to the pads, any recommendations? 328whp, ~3100lbs

After doing some testing this afternoon, everything as far as the brake booster appears to be working correctly. For example, even if I pump the brakes while in boost ("using up" the vacuum in the booster) momentarily letting of the gas or even a quick shift seems to be enough to bring back a normal pedal feel.

 

I used PF 01 pads last season on 245 Ecsta XS's and was very happy with them. I'm running them again this year.

 

Yes I've done the same experiment, left foot brake while in boost until all vacuum is used up then with foot still on the now rock hard brake let off the gas, it takes about 1/4 second for the pedal to return to normal and the car to start slowing.