61 replies to this topic

[SaulSpeedwell]

A lot about this just doesn't make sense.  In an effort to be helpful despite that I can't make a solution out of this pile of data, I offer the following:

 

-Some things will be everchanging throughout this entire period - like pad thickness.  So, the only time you are able to reliably isolate knockback is when everything is at full thickness. 

-If it comes and goes, it can't be fluid boiling. 

-If the pedal is ever firm, it can't be "air".

-If you drive aroud the paddock at 20 mph dragging the brakes until all 4 sizzle when you spit on them, then all 4 brakes are "working".  Air in one branch won't allow this.

-Brake Engineer Rule Of Thumb - "It's never the prop valve".  This prop valve might leave you with overworked or underworked rears, but it can't do anything to the fronts, and it can't cause intermittent pedal issues.

-If calipers caused it, but putting old cali involpers on didn't fix it, one of the 10 other fixes caused the same problem OR - IF your problem is really knockback, the process of taking the old sticky calipers on and off and the new fluid involved, has now made the old calipers as knockback prone as the new ones.

 

It all sounds like knockback, and this is the one thing that can happen despite that everything in the system is hydraulically, mechanically, and vacuum-ically perfect - except you say when you have all new stuff on, you still get a low pedal toward the end of the session.  If it were my car, I would shim it so knockback is NOT possible.  I would drive it until the complaint appears.  I would then measure the clearance at all 4 corners.  I would hit the pedal a couple times.  Then I would "pry" the calipers back however far they can go and hit the pedal.  If it feels like what is happening on track, then your prob is knockback and you can quit spending time and money on anything that won't address that specific issue.

 

Another thought:  You say the booster helped.  Make sure the check valve and hose going to the intake mani are working and not leaking, which could wonk up your pedal feel intermittently enough to make you think all sorts of voodoo is happening.  I think you mentioned fast idle after a motor swap - fast idle can starve you of vacuum - fix the fast idle - it is easy to do, just fiddly, and I have no idea why so many people suffer with incorrect idle speed, making tuning a PITA and making shifting and braking and brake balance a potential issue.

 

Let us know what you eventually find!

[SaulSpeedwell]

And another thing - make sure the master is fully retracting and not being held partially "stroked" because someone messed up the yoke adjustments and such. 

[Todd Green]

Thanks Saul for taking the time to write a detailed response.  WRT idle, that is fixed.  The timing was off on the motor and there were some vacuum leaks that we found with a smoker.   The check valve is new and works.  (Also tested by taking a working one from another car and put it on mine to triple check.)  Yes the arrow is in the correct direction.

 

I agree with knockback, but what is weird is you come off the curbs, pump the pedal half way down the straight (several times until it is high and firm) and it'll still drop at the end of the straight even though you hit no curbs.

 

There is no vibration in the car, hubs seem good, etc.  But it's as if something is just pushing the front pads back as I go down the straight.

 

I'll check the master, booster, and pedal adjustments again.  Any details on where you get/make shims?

[Todd Green]

While searching around for shims...I hit this thread....worth reading:

 

http://mazdaracers.c...e-pad-kickback/

[davew]

I see 2 things sticking out

 

<<I agree with knockback, but what is weird is you come off the curbs, pump the pedal half way down the straight (several times until it is high and firm) and it'll still drop at the end of the straight even though you hit no curbs.>>

 

The pump up should occure just before normal braking, as late on the straight as possible.

 

<<There is no vibration in the car, hubs seem good, etc.  But it's as if something is just pushing the front pads back as I go down the straight>>

 

Almost sound like a rotor is warped and causing the knock back issue. It really does not take much warpage to cause a problem at race speeds.

 

I would put new high quality rotors with new pads and see if the problem happens again.

 

 

And listen to Saul. He is almost always right.

 

Dave

[Todd Green]

I do pump just before braking normally.  I wanted to do a test to see if I could get "knock back" without being on the curbs.  I can.  This problem has been happening for some time; so I've been through multiple sets of rotors (Mazda, Centric, etc.).  There is no feedback/shuddering in the brake pedal like you get with warped rotors (or pad deposit buildup that many mistake for warped rotors.)  The problem happens with a brand new set of rotors. I've had other very experienced drivers (e.g. a National Champion)  drive the car and they sail it off the track in the same turn as the video and come off the track wide eyed and say, "You need to get that fixed.  Your car is evil."

 

Again, all good suggestions.  Unfortunately all things I've already tried.

 

I have a test day this Sat...I'll report back then.

[AW33COM]

I wanted to do a test to see if I could get "knock back" without being on the curbs.  I can.

 

The "knockback" you "get" on the front straight does not have to be knockback at all.   That could also be a textbook example of air in the system.   Think this through: You pump the brakes early on the straight.  Pedal feels fine.  You brake few seconds later and you have no brake.  When you pumped the brakes the pressured was built as the air went down.   Then you drove for few seconds, the pressure in your brake system dropped as air went up. 

 

You're going to ask me, how would there be air if I'm not leaking any brake fluid?  Very simple.  You changed every component, you replaced and bench bled the master brake cylinder.  There are tons of things that can happen between bleeding your MBC on the bench and installing it.  One of them is: letting air into the cylinder when hooking the brake lines without a second person squeezing the brake pedal gently. I'm not saying that's it, but ther is still a range of unanswered questions for me.

 

Anyway, at this point I could write a book about brakes, but I hate guessing the brake system.  Your problem is not magical, all you have to do is segregate your system problems from on track knock back issues and I can help you from there. 

[Todd Green]

But if there is air in the system, wouldn't I have at least a spongy pedal 100% of the time best case?   Worst case, I'd have a long pedal all the time. The brakes have probably been bled at least 10 times since I first started having this issue.    The air can't go away.  So even pumping the pedal would still leave it spongy in every corner, and I don't get that.  No?

[AW33COM]

So even pumping the pedal would still leave it spongy in every corner, and I don't get that.  No?

 

In general you are correct, but it depends where the air is, how much, how it gets there and etc.  But like I wrote I don't think that's it, I was only trying to show you the pumping on the front straight does not have to mean break pad knock back. 

 

Anyway, test the system in the way I posted the first time, plus turn the front wheels all the way right and left to make sure the break line is not pressed against anything.  If you can't replicate the problem you will be 95% sure it's knock back related and then you'll test for that.  It's easy. 

 

Later on I'll post how to test the booster, so you don't have to every worry about that guy. 

[AW33COM]

As promised, here is a simple way to test the booster along with few tips.   I'm 100% sure the booster is not your problem, but knowing that you changed it and knowing it costs $500, I'm going to post very simple brake booster tests, so that nobody in the future will change one unless they have to.

 

Few facts first:

1.  Brake booster NEVER creates a soft pedal.

2.  Brake booster NEVER creates a low pedal.

3.  Vacuum problems (PCV and other things people change when the pedal is soft) would NEVER create a soft or low pedal.  Tip: nice way to test for vacuum problems in the paddock is to pull a healthy Miata alongside and moving over the vacuum hose.

4.  Brake booster always creates.....that's right, a HARD pedal.

 

 

Three brake booster tests:

Test 1. (Operation).
Engine Turned Off.  Pump the brake and hold the pedal down.  Start the engine.  Pedal must go down a bit.  If the pedal does not go down booster is not working.

Test 2.  (Air Leak Check)
Run the engine for 5 minutes.  Turn it off.  Press the pedal about 3 times.  Each time you depress it, wait about 6 seconds.  If the brake pedal "goes" in less and less with each pump the booster is air tight.  

Test 3.  (Air Leak Check under pressure)
Start the engine.  Pump the brake.  Stop the engine and hold the pedal for a minute +-.  If the pedal position does not change all is good.

 

This is not a complete test for the booster, but a good start. 

 

 

[LarryKing]

WAG - is the Iron Canyon sensor plumbed into the hydralics? Have you tried removing it?

[Bench Racer]

I'm not a real bright guy, but I'm smart enough that with an on going issue as you have with your brakes (had I done the work you have done) I would let the Pros at a prep shop do their thing.

[Tyler Dahl]

^^^^ that's funny!!!!!

[Todd Green]

$500?  Who-wha?  It was like $180 or something close to that.  Can't remember the exact amount.  I did follow the FSM procedure for testing the booster (basically what you posted IIRC).  The old one checked out fine.  But it was to the SNAFU point.  I did swap the vacuum hose (twice), even though the old one tested fine as well. I agree that it shouldn't have affected the pedal dropping, but it was the very last mechanical piece in the system that I hadn't replaced. "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."  and all that.
 

Bench - That's like the build vs buy argument.  Some want to learn about the cars and do things for themselves.

 

88 - The sensor comes off the master  (sensor isn't from ICM FWIW, but the banjo adapter is).  Problem predated the sensor by a long time.

[Johnny D]

Ok, let get serious. I guess you need to hit the YouTube button.

[Bench Racer]

Bench - That's like the build vs buy argument.  Some want to learn about the cars and do things for themselves.

[SaulSpeedwell]

As promised, here is a simple way to test the booster along with few tips.   I'm 100% sure the booster is not your problem, but knowing that you changed it and knowing it costs $500, I'm going to post very simple brake booster tests, so that nobody in the future will change one unless they have to.

 

Few facts first:

1.  Brake booster NEVER creates a soft pedal.

2.  Brake booster NEVER creates a low pedal.

3.  Vacuum problems (PCV and other things people change when the pedal is soft) would NEVER create a soft or low pedal.  Tip: nice way to test for vacuum problems in the paddock is to pull a healthy Miata alongside and moving over the vacuum hose.

4.  Brake booster always creates.....that's right, a HARD pedal.

 

 

Three brake booster tests:

Test 1. (Operation).
Engine Turned Off.  Pump the brake and hold the pedal down.  Start the engine.  Pedal must go down a bit.  If the pedal does not go down booster is not working.

Test 2.  (Air Leak Check)
Run the engine for 5 minutes.  Turn it off.  Press the pedal about 3 times.  Each time you depress it, wait about 6 seconds.  If the brake pedal "goes" in less and less with each pump the booster is air tight.  

Test 3.  (Air Leak Check under pressure)
Start the engine.  Pump the brake.  Stop the engine and hold the pedal for a minute +-.  If the pedal position does not change all is good.

 

This is not a complete test for the booster, but a good start. 

 

 

 

OK.  I swear swear swear I am not trying to be "that guy" on the Internet that argues over nothing based on some esoteric counterpoint, or just because I think I am the Grand Poobah Of All Braking Knowledge.  Please believe me.  I learn things about braking every day from all sorts of people, and I share the following rambling in an attempt to be helpful.  I am not trying to be argumentative or crap on your suggestion, which has a lot of merit and sound reasoning.

 

What you say above is (more or less) true ON PAPER.  If we put force and travel sensors on the pedal, your test protocol will succesfully exclude the booster, most of the time.  But - I have seen countless - seriously I cannot count how many times - brake complaints that would pass your tests and that are still directly attributable to the booster.

 

Why?  Because we, as human beings, totally suck at accurately perceiving and reporting what we THINK we feel in our foot.  I have had people - car enthusiasts, mechanics, engineers, racers - swear on their loved ones' graves that they have "spongy pedal", "no brakes", "brake failure", "sudden fade", etc., never thought to be a booster issue, all of which turned out to be the damn booster.  When power brake assist comes and goes, our foot and our brains interpret it as "brake failure".  

 

In short, if vacuum (and thus power assist) comes and goes, then pedal hardness SEEMS to come and go.  Going from no assist to lots of assist feels the same as "spongy pedal".  Guys that track or race turbo cars with no vacuum pump will know what I mean - you get to the end of the straight, pedal feels like a brick but you aren't stopping, then suddenly the pedal gets "sucked" down and you stop like mad as power assist is restored as vacuum builds. 

 

Some anecdotes:

1.  Myself - using something akin to your test protocol above - was completely convinced that I did not have a booster problem on an old Datsun Z I was restoring.  I bled and bled and bled and drove with power assist disconnected and pumped the brakes with the engine off and installed pressure gauges and replaced things and made myself miserable, until I discovered the reaction disc fell out of the booster during cosmetic "restoration" of the booster.  I would have sworn to you that it wasn't a booster problem, but it was.  Why was I wrong?  Because- again - we humans are TERRIBLE "transducers" when it comes to perceiving and reporting what we THINK we feel in terms of pedal force and travel.  I thought I was above that and smart enough to know better than everyone else - but I wasn't.  I was completely wrong.  What I thought I felt and what I thought was happening was dead wrong, despite 15+ years in brake engineering.  It was the damn booster the whole time, and I refused to believe it.  And thus, I spent 6 weeks wasting $100+ of premium fluid and compromised paint and irritated skin racking my brains out trying to figure out why my brake feel was soooooo intermittent and soooooo bad when there was no problem before I decided to cosmetically "restore" everything.  Still pisses me off today!

 

2.  A customer of mine SWORE SWORE SWORE SWORE TO GOD that he had "spongy pedal", "air in the lines", "gotta pump the brakes to stop after a few laps", etc.  Get this - NO BOOSTER at all on this car.  He bled everything, adjusted the bias bar, added cooling, upgraded fluid boiling point, installed pressure gauges, etc.  Same complaint.   Without really telling him what I did, I did nothing more than raise the pedal 1/4" or so with a yoke adjustment.  No complaints in 3 years since.  How can that be?  It isn't possible physically - it is only possible "mentally".

 

My only point here:  Humans, even experienced racers, brake engineers, and people that are both - really really suck at perceiving and reporting what is happening under their foot.  We detect changes in feel and performance very well - but we are terrible at inductively (or deductively) determining what it is we actually feel, much less why we feel it.

 

In any case - I'm very curious to hear if the problem gets fixed and what is reported to be responsible!

[Ron Alan]

Saul...please be that guy!

[Todd Green]

Ok....you're not going to like this, I don't like it, but I don't/didn't have time to throw one thing at a time at the problem.  (Nats in a few days.)  Of course changing one variable at a time is ideal.  Having working brakes is more ideal.   So I did the following:

 

1) Rebuilt my front calipers (Mazda kits).  One of the pistons was heavily pitted, rough with some substance applied to the sides of it.  (Wanted to take pics, but I think my garage mate chucked it in the trash before I could get to it.  I was seriously thrashing to get the car ready in time for my sessions).  Other looked fine.

2) Replaced the proportioning valve with new Mazda valve and the two hard lines from the master to the prop valve

3) Blew out all the hard lines with compressed air one at a time.  No foreign objects came out and nothing seemed obstructed.

4) Put on all new braided lines (I found a set I had, so why not while I was at it.  The old ones are going on a "chumpcar".)

5) Bench bled the master, pressure bled the system (twice)  Motul 600

6) Oh yeah, did verify that all 4 corners had the right caliper, both before and after the changes.  R's on right, L's on left.

 

I kept old front pads on to maximize piston travel.  Probably 1/4 to 1/3 left.  Definitely at (or beyond) the limit as to what I'd run before putting new pads on.  Rear pads have two race weekends on them.  Virtually new.

 

Ran 3 or so laps as I normally would with no issues (MMP West Track).  Then I tried running over the exit curbs (huge bump and gators) in just 3.  Didn't touch the gators in 4 and didn't pump the pedal for 5.  It probably went 1/4 way down before engaging.  Next lap I did the same in 3 and added running over all the gators in 4.  Pedal probably went down 75-80% of the way, but I still had some front brakes.  Both of these are right side and the car is pretty loaded in 3, just a bit in 4.  Tried just running over the gators only out of 4 and the pedal went down similar to just running over them in only 3.

 

Next I tested running over the gators coming out of 9 (left side) and had zero pedal issues (i.e., high/firm pedal without pumping) in the next (hard) braking zone.  However the car isn't loaded at all going over those.

 

I redid these tests several times and they were very repeatable.   So I don't know the right/left differences.  I did check wheel bearings (just rocking tires/rotate 90 rock again) and everything was tight.  But my bearings do have 14 race days on them or so.  Rights take more of a pounding, so perhaps there is a bit of slop that causes more knock back on them?

 

Anyway, as long as I gave the pedal a pump or two I had good brakes in every corner.  (Though I "ruined" a bunch of corners by tapping the pedal a zillion times, since I've become so used to it, I'd just start doing it subconsciously half way down the straight as I was checking other stuff related to the new motor.  Grrr.)

 

So it seems to be fixed (*).  For how long I don't know.  If it shows back up, I'll post, otherwise figure one or more of the steps above solved the issue.  Thanks everyone for their help and input.  See (some of) you at MMP in a few days!

 

* Well, fixed "enough".  I still don't like that running over left wheels on gators gives me different results than right.  I'd like to build a set of gators for the paddock so I can do more controlled testing.  Should be easy to do with a 2x8, but that'll have to be down the road.

[Tom Hampton]

Whoa. Hang on a sec. You said the calipers were new... One set from rock auto the other from Mazda speed. So, this new caliper had a BADLY pitted piston?