61 replies to this topic

[Todd Green]

I've been fighting my brakes for ages now.  I had the car for a full year with zero issues.  I never even got pad knock back.  I decided to change the calipers and that's when it all started.   Every so often the pedal just goes to the floor.  You have (virtually?) no front brakes and the rears lock up.  Here's the vid (Sorry for the logos, it was faster to just use my template):

 

 

Here is a screen shot of the data on from a lap where I have the issue and a "normal" lap.  (Red == issue, blue == normal).  You'll notice that I'm maxing out the brake sensor.  The little blimp is where I tried to pump the pedal to no avail.  Sensor is using the Iron Canyon adapter and is mounted on the line near the front of the master facing the fender.

 

I've replaced *everything* in the system other than the hard lines.  Master, booster, prop valve (with a used "known" working one), calipers twice.  I bench bled the master, bled the lines til I was blue in the face, squeezed the rubber on the pins.  All pins are greased, calipers slide etc. etc. etc.

 

The thing is it doesn't do it every lap.  It's definitely knock back related as it gets worse as pads get thinner.  If I pump the pedal before braking like 5 times with really short/easy strokes  (ok...Webb no jokes!) I almost always have a pedal.  Every once in awhile it'll go like a 1/4 way down before engaging.  It also never does it in the paddock, the pedal is high and great.  Even the first laps of any session are great.  So don't know if it is heat related.  I use Motul 600.  Never have air, never get a soft pedal.  Like I said, it can go to the floor one lap and be fine the next corner and next 20 laps.   Generally in a given session it'll do it 2-3 times.  (If I'm behind someone I always pump the snot out of it down the straight and brake early. Makes for "fun" racing. I've never had issues when I concentrate on pumping it.  Looking at my data, I tapped the pedal 4-5 times, but a bit lighter than normal in the vid above.)

 

All parts (other than calipers, one set was from Mazda, the other from Rock Auto) were new and from Mazdaspeed.  I do have a new prop valve and hard lines from the master to the prop valve that I'm going to try next.  I'm at my wits' end and would really love to get this figured out before Nationals.  Help?

 

[Alberto]

Isn't there some sort of one way check valve between the brake booster and the intake manifold for vacuum?  Have you checked that and the associated lines?

[Todd Green]

Isn't there some sort of one way check valve between the brake booster and the intake manifold for vacuum?  Have you checked that and the associated lines?

Yes, checked (was fine), replaced anyway with a hose from another car for a session, still had the problem.  Bought new hose from Mazda for ***** and giggles, still didn't fix it.

[Bench Racer]

I've been fighting my brakes for ages now.  I'm at my wits' end and would really love to get this figured out before Nationals.  Help?

"We're too busy at tracks actually racing our cars, doing well, and having fun."

 

If it were me, I'd stay home and get the brakes issue solved before I did one of those T1 deals.  

[Johnny D]

"We're too busy at tracks actually racing our cars, doing well, and having fun."

 

If it were me, I'd stay home and get the brakes issue solved before I did one of those T1 deals.  

You may want to consider the opposite.

J~

[Alberto]

Yes, checked (was fine), replaced anyway with a hose from another car for a session, still had the problem.  Bought new hose from Mazda for ***** and giggles, still didn't fix it.

OK.  What's this "prop valve" you are referring to?

[Kevin Anderson]

It's the curbs. When the car goes over those bumpy curbs the vibration cause the rotor to wobble I believe, and that spreads open the calipers. You have to learn to give the brakes a slight pump after those curbs before you need them

[Keith Novak]

OK.  What's this "prop valve" you are referring to?

brake proportioning valve

[Todd Green]

It's the curbs. When the car goes over those bumpy curbs the vibration cause the rotor to wobble I believe, and that spreads open the calipers. You have to learn to give the brakes a slight pump after those curbs before you need them

 

From OP "If I pump the pedal before braking like 5 times with really short/easy
strokes  (ok...Webb no jokes!) I almost always have a pedal."  I know what knock back is.  I can hit the pedal once (or twice) after the curbs and it'll still do it.  Trust me it isn't normal.

[AW33COM]

Jack up the car (make sure car is in 100% race setup) so that all 4 wheels are up and it's safe and steady.  Get in, and start driving up in the air as if you were in a session.  You don't need to rev the engine, just keep the speed the same.  Do this until you're able to replicate the problem, or you verify it does not happen in this configuration. 

 

The test above will achieve 2 things:

1.  It will test your entire brake system. 

2.  Verify if the problem is related to lateral forces (pad knock back) or not.

 

Post the results and we can take it from there.   Pad knock back can occur from many different things, but let's verify your system first.

 

* The brake fluid you're using, is that fluid from the same container each time.  I don't think that's what it is, just trying to figure out what you did not replace.  Btw, should have never replaced the booster, it is expensive, but there are great tests for it that would rule it out.  

[Johnny D]

Jack up the car (make sure car is in 100% race setup) so that all 4 wheels are up and it's safe and steady.  Get it, and start driving up in the air as if you were in a session.  You don't need to rev the engine, just keep the speed the same.  Do this until you're able to replicate the problem, or you verify it does not happen in this configuration. 

 

The test above will achieve 2 things:

1.  It will test your entire brake system. 

2.  Verify if the problem is related to lateral forces (pad knock back) or not.

 

Post the results and we can take it from there.   Pad knock back can occur from many different things, but let's verify your system first.

 

* The brake fluid you're using, is that fluid from the same container each time.  I don't think that's what it is, just trying to figure out what you did not replace.  Btw, should have never replaced the booster, it is expensive, but there are great tests for it that would rule it out.  

For the fronts you don't even need the engine running. Just 2 people spinning the wheels and making engine noises.

J~

[Ron Alan]

Jack up the car (make sure car is in 100% race setup) so that all 4 wheels are up and it's safe and steady.  Get in, and start driving up in the air as if you were in a session.  You don't need to rev the engine, just keep the speed the same.  Do this until you're able to replicate the problem, or you verify it does not happen in this configuration. 

 

The test above will achieve 2 things:

1.  It will test your entire brake system. 

2.  Verify if the problem is related to lateral forces (pad knock back) or not.

 

Post the results and we can take it from there.   Pad knock back can occur from many different things, but let's verify your system first.

 

* The brake fluid you're using, is that fluid from the same container each time.  I don't think that's what it is, just trying to figure out what you did not replace.  Btw, should have never replaced the booster, it is expensive, but there are great tests for it that would rule it out.  

Please stop...

[Ron Alan]

Todd,

 

I read your first post and unless I missed it, you do not mention changing rotors or pads(I know you do say everything but I dont want to assume). Any chance you have rotors that have gotten so hot they have lost their integrity? Though I may not be using a correct analogy here...I see what over heating a saw blade does to it. Before applying a load, my 10" table saw blade will spin true and free. As I start pushing a piece of plywood thru and the blade gets hot it will "chatter"...basically vibrate cyclically(creating a larger then smaller kerf). My better blades...ones that cut and don't over heat do not do this.

 

I've often wondered if rotors(especially of questionable original integrity...cheap)will do this on a regular basis once they've gotten hot enough one time to change things. Hypothetically...you have a long hard brake into a slow corner, as you release your brake, the heated bad rotors vibrate out of true as they spin pushing the pads away. As they cool quickly enough they spin true again before the next brake zone...if all 4 rotors are doing this it would make for a really long pedal!! And then knockback caused by other sources only adds to the problem. In the paddock, the pedal stays where it should be because you have cooler/slower turning/true spinning rotors??

 

If you had the problem on the first session of new rotors and pads...then my theory isn't worth   ! Unless maybe you have been using the same brand of rotor since your problem started?? Hope you get this solved soon!

[Caveman-kwebb99]

From OP "If I pump the pedal before braking like 5 times with really short/easy
strokes  (ok...Webb no jokes!) I almost always have a pedal."  I know what knock back is.  I can hit the pedal once (or twice) after the curbs and it'll still do it.  Trust me it isn't normal.

 

No joke, I think you need to take this car to the Temple there in SLC, and get the deamon out of your car, especially with all those short strokes!

 

Ron I think is on to something!!!!!!

[Todd Green]

Ron appreciate the input.  I've changed pads and rotors several times.  (Due to normal wear.)  I have had the issue at the end of the first session after a change.

[Keith Andrews]

Disclaimer: I'm a driver that works on my own car not a pro.  It sounds like you have covered the bases.  Definitely frustrating with the potential of ruining your day.  

 

From your video you should definitely be getting pad knock back.  That is normal and desirable, free spinning wheels are good.

 

You could run around the track at 3/4 pace, staying off the curbing, so that you aren't dealing with varying degrees of knock back.  It might make it clearer in your mind what exactly your brake pedal is doing. 

 

I've rebuilt, changed all the seals in a master cylinder and it still did not work as well as I liked.  I would get a randomly soft brake pedal in the paddock and on track.  Then I bought a new one and my problems went away.  If there is no air in the master cylinder and lines I would revisit the master cylinder.  My .02

[Todd Green]

Thanks Keith.  Master is brand new (not rebuilt) from Mazda.  Not to say that new parts can't be bad, have air etc.  Here's a snapshot of me pumping the pedal on the two laps.  You can see on the problem lap I didn't press the pedal with as much force, but I pressed it 5 times.

 

1) I should only have to hit the pedal once or twice

2) After hitting the pedal, I shouldn't get "knock back" down the straight (since I'm not on any curbs, the wheel is straight etc.)

3) If the brake pressure sensor is showing that I have max PSI, why is the pedal going to the floor?  This seems to indicate to me the problem is after the master cylinder.  (Or in the other line coming from the master.)

 

[Keith Andrews]

Agreed and I follow your logic.  The fact that you aren't losing fluid (leaky calipers, lines, master cylinder) or getting air in your lines, makes it sound like mechanically the system loop is sound.  

 

Regarding the data and pressure indications, I just don't know enough about the system plumbing and sensor locations, to comment.

 

I do know of a fellow driver that had a similar problem with the same pedal to the floor and or rears locking up.  I'm near positive the master cylinder was the problem, where else is there a split in the system front and rear? 

 

If the caliper change was the beginning of the problem I guess that would be another place to revisit.

[Adax]

I agree with Keith. I think you've got a bad seal in the front circuit of the master. Sometimes is seals, sometimes it does not. If you are not losing fluid, the caliper seals must be ok. The only place fluid can bypass a seal and not be lost is the cylinder, I think. Then again, if the behavior was exactly the same before the MC change, I think you can rule it out.

 

I assume the front hubs are ok?

 

The best way to figure this out would be to place a pressue sensor in each of the front lines, if that's possible.

 

Was the pin that connects the pedal to the booster rod good when you replaced the booster?

 

Probably wasn't much help.

AC

[Todd Green]

Keith...yep started by replacing calipers. Didn't fix it.  Went to the original calipers that had no issues, didn't fix it.  The other area where you get a split is in the proportioning valve.  I have a new one to put in, which I'll do on Sat.

 

The general rule is if things are working and you make a change, the change 99% of the time broke it.  I'm still baffled that it wasn't the calipers.  Or at least changing calipers didn't fix it.

 

I'll have to say that when I replaced the booster, things got *much* better.  For awhile.  It seems that things have reverted back to where they were before which is that you have to pump the snot out of the pedal to have brakes.

 

I also have considered adding more brake sensors so I can monitor each line, but they are expensive, a pain to add, and useless for day-to-day data acq since we can't adjust our bias.  But I'm to the point I'll probably get another one so I can see front/rear bias.

 

I assume the pin was good...didn't have any issues with it.  Is there something specific I should be looking for?

 

Edit: Front hubs are fine.  Replaced less than a year ago, no play in wheel when rocking it and then rotating 90 and rocking again.

 

Thanks again for everyone's input.