61 replies to this topic

[Caveman-kwebb99]

from all that has been said, my pee brain says you have some kinda dirt or object floaitng around in the line, maybe its hung up at the proportioning valve.  when it floats over just the right way everything is good, when it floats the wrong way and clogs somthign up a bit things go haywire. 

 

If it was me, I would disconnect every line one by one and blow them out then flush new fluid all the way back through and while you are at it pull the porptioning valve and check it over rplace or whatever, and when the valve is out I would blow that part of the lines out as well...

 

I would then put it all back together and fill and bleed it with cheepo fuild a few times, then flush it through with good fluid before taking it to a track day, I wouldnt want to show up at a race with unknown brakes.

 

BTW I had a similar issue this spring at the Drivers Ed at cota, I had Gilfus swap the master to a new one and bleed, it was better but not great, it has gotten me in a great habbit of always checking those brakes before the brake zone, the problem has pretty much worked itself out now, but I notice it comes back a bit when my pads are low.  I nevr had this issue before we rebuilt the car over the winter, we had all brakes off and subframes replaced etc. I really think we had some dirt in the lines that was playing havoc with the system, i think it has wrked its way through on my system, but I have not been on Gators since Cota, regualr curbs dont knock my pads back like tha gators.

[Todd Green]

I like the idea of trying to blow out the lines.  I've not done that.  I'll put the new prop valve in after doing that.  Thanks Kyle.

[Caveman-kwebb99]

I meant with air... but maybe thats what your assuming i meant???  I might even go as far as sending some solvent through the lines followed by more air to act as a cleaning agent.

 

I know its sounds crazy, but you have tried what seems to me to be everything else and it hasnt changed which makes me think there is something lingering in your lines...

 

Dont let bench know I am giving tech advice as he has a corner onthat market

 

I am also curious where is the proporting valve located todd? I have never gone looking for one and always assumed it was proprtioned at the master unit...

[Ron Alan]

Since you can get a good pedal once you pump...what is relieving or moving the fluid back into the Master?? I'm still of the theory it is vibration related at the wheels...gators, curbs, wheel balance, warped rotors....something that is shaking enough to cause more than normal caliper release/retraction. We know the master is not a one way valve as we can push fluid back into the reservoir when we compress the caliper pistons...and air would always create a soft longer pedal and never get better.

[Phil Mather]

It's a long shot, but have you reversed the lines out of the prop valve sending front pressure to the rear and rear pressure to the front?  Just something simple enough to check.  

[Mark McCallister]

Todd, you said you replaced everything but the hard lines, but you didn't specifically call out replacement of the soft lines between the car and calipers, unless I missed it (quite possible in a 2-page thread!).  Just checking...I've been wondering myself what the preventative replacement lifecycle is for those.

[SaulSpeedwell]

Install full thickness pads and rotors OR shim the pads so there CANNOT BE any pad knock back.  Fixed?  Then knockback was the problem. 

 

I was never very good at the "pump the brakes" routine, and considered it something that made me distracted and inconsistent at best, and kill my laptime on occasion at worst.  So - if it was a race that mattered, I ran pads that were "blueprinted" to be just the right thickness and I kept shims on hand to compensate for wear.

 

In any case, this will absolutely determine if knockback is your problem or not - just shim until the caliper barely slides over the pads, and then retest.

[Caveman-kwebb99]

shit just ask the brake engineer!!!!  why didnt we all think of that instead of making a bunch of guesses??????  going back into my cave now

[Todd Green]

Wow lots of stuff

 

1) Yes, I knew you meant air.  Compressed at that, I assumed you didn't want me to blow the lines.  We'd need DD lung power for that to be effective.

2) Yes replaced the braided lines with new Goodrich ones.

3) I did check the lines to see if they are reversed.  They are all a specific length and should be good by comparing to other cars

4) By the end of a session with new pads I've had the pedal go low.  IIRC, I don't believe all the way to the floor, I don't think there is enough travel in the system for that to happen.

5) There are no vibrations in the car.  It happens with brand new rotors etc.

6)  This is the prop valve.  On the firewall near the master/booster (pic found on the Net, not from my car.)

 

 

What I still don't get is what has changed?  Only the calipers were changed (initially).  In theory nothing should have been contaminated, etc.  Sure some fluid dripped out, but I made sure the reservoir was always full etc.

 

This is the most evil problem I've had to solve in 15+ years of being on track.  After just swapping a motor last week, not being able to get my idle below 1300 rpms (until the computer retards the timing) is quickly vying for the #1 spot though.  Glad Nats is still 3 weeks away.

[Randy Thieme]

You data plot shows brake pressure.  How is the brake pressure sensor fitted?  I remember sitting in a data acquisition seminar and they mentioned something where if a brake pressure sensor is not fitted correctly it can trap air which won't come out when bleeding.

[Todd Green]

It uses the Iron Canyon banjo adapter.  Problem started long before I added the sensor.   Appreciate the input though!

[Johnny D]

I'll ask the donkey question.

 

Your using Dot 4 right?

J~

[Todd Green]

I'll ask the donkey question.

 

Your using Dot 4 right?

J~

Motul 600.

[Tom Hampton]

Todd-

 

Does it happen driving on the street?  Woud it be possible to drive down the street under whatever conditions cause the long pedal. Then coast home and use the ebrake to bring the car to a stop.  Jack the car, and inspect the front calipers for knock back. ??? 

 

Of course, you can't take the wheels off, because you might wedge the caliper apart.  But, a pedal THAT long should be easy to tell the difference even with the wheels still on.  The calipers should slide back a forth a ton on the pins. 

[Keith Novak]

How high do you have your brake pedal set?  I started having brake knock back after switching to nicely remanufactured calipers and the pedal goes almost but not all the way to the floor with heavily warn pads after exiting a corner on the curbing.  Scared the crap out of me a couple times but I've picked up the habit of taping the brakes early to extend the pistons since then.  I have my brake pedal set as high as it will possibly go.  If it wasn't set as high as possible I could definitely see them bottoming out.

[Todd Green]

Car isn't street legal and has no ebrake.  It *never* happens in the paddock.  What I need to do is see if I can get it to happen without using the gators, but track time is rather limited and I hate wasting it on stuff like this.  After Nats I'll have plenty of time to test and will give up a session without using curbs.

 

Pedal is pretty high.  High enough that during a rain race with new pads I couldn't heel-n-toe because I couldn't press the pedal hard without locking up due to the slippery conditions.  I was actually thinking about lowering it. (Before all these issues started.)

[lm11m]

Not trying to be a wise guy, but when you replaced the calipers you do have rt. on rt. & lft. on lft.? (bleeders on top) Over the years I've seen this happen many times to do it yourselfers. Brakes appear to bleed out but there is still air trapped in top of caliper ( where bleeder should be), a few pumps gives pedal but  when pedal is released air has to be overcome again by pumping. Possible simple mistake & an easy fix!

[LarryKing]

I don't think it's possible to mount the front calibers on the wrong sides. The two mount bolts are different diameters, and the rear calipers are impossible to switch.

[davew]

I don't think it's possible to mount the front calibers on the wrong sides. The two mount bolts are different diameters, and the rear calipers are impossible to switch.

 

It is possible if you flip both caliper and bracket

 

Dave

[Todd Green]

I remember someone posted about having calipers flipped some time ago.  I'm pretty sure they are correct, but I'll add that to the quintuple check list when I'm at the track next week (where the car is stored.)  I do remember looking for the R and L stamps when I installed them, but that was last year and checking is quick.  Bleeders are definitely on the top and not the bottom of the calipers.