Help: Stumped about brake behavior on track

[parsonsj]

I've got my car back from the Eyes on Design show in Detroit, and I want to get to the bottom of the odd behavior I experienced at Road Atlanta. The brakes work great for normal street driving; no complaints there. I've been blaming my pedal linkage ... but now I'm not so sure.

I'm using Wilwood 6 piston Superlites with 14" rotors in the font, and 4 piston Dynalites with 12" rotors in the back. The car has very little weight transfer during braking. (very low, 60% anti-squat, high rate springs).

Anyway, I've got a balance bar with separate masters and brake line pressure gauges. So I can see the line pressure, and easily adjust the ratio. Right now in my garage, I can push all day long on the pedal and get 1100 psi both front and back, with the front a bit higher than the back. I figured that would be a good starting point since the rotor and calipers are more capable up front.

(enough already, what's the problem?) During track events, the pedal gets soft and tends to stay low. If I pull it back with my left foot, the next brake will be very solid, but it will slowly go lower until I pull it back again. And the line pressure in the front is much lower: maybe 50-60% of the back.

This doesn't take long ... on Road Atlanta this started happening within 8 laps or so. So ... am I boiling fluid? Water or air in the front brake lines? Is there a secret to balance bar setup I might be missing?

Thoughts?



jp

[2yellow69]

something to look for besides what you already mentioned is brake fluid residue on the caliper adjacent to the pistons. awile back i saw a hot caliper seep brake fluid from the piston seals. i seriously doubt wilwood calipers would have this problem, but you never know. i think it was caused by boiling fluid. what brake fluid are you using? if it's dot 3 you might want to step up to dot 5 if you're going to run on the track a lot. have you had to add any fluid? if not every thing i just typed is hogwash! hope this helps.
matt

[2yellow69]

another thought could be the wrong compound brake pads. or rotors that swell under repeated hard braking causing the pads to hang a little and not be able to cool down. or.... man i hate brake problems!! i feel your pain when it's not a straight forward repair problem.
anyone else have some thought on this?

[TitoJones]

Are you sure it isn't a pad knock back issue? If you pump the pedal does it better? Have you had to add fluid after the track day?

Tyler

[parsonsj]

Pad knock back may very well be the problem. Yes, pumping the pedal makes it better, and no, there's no fluid loss. I also noticed a "ticking" noise on right turns as the day went on ... if it's a wheel bearing, that will make a pad knock back issue worse, right?

Also ... it occurs to me that my front brakes are still using the original fluid, which is over a year old now. The rear brakes were flushed and changed when I narrowed the rear axle housing. Perhaps the front brakes have a little water in the fluid?

jp

[6'9"Witha69]

Pad knock back may very well be the problem. Yes, pumping the pedal makes it better, and no, there's no fluid loss. I also noticed a "ticking" noise on right turns as the day went on ... if it's a wheel bearing, that will make a pad knock back issue worse, right?

Also ... it occurs to me that my front brakes are still using the original fluid, which is over a year old now. The rear brakes were flushed and changed when I narrowed the rear axle housing. Perhaps the front brakes have a little water in the fluid?

jp

Sounds like you figured out a good starting point. Yes a loose/bad bearing will create/worsen knockback. The old fluid could also contribute, but check the bearings and suspension first.

Another thing you stated in the first post was that you would lift the pedal with your left foot. Do you have a return spring on the pedal? Wilwood is very specific with their MCs that the cylinder fully return but do not get smacked back into place.

The second bullet point on the link below says the following:

Be sure the pedal and push rod are adjusted to allow the master cylinder piston to fully retract against the snap ring at the back of the master cylinder bore when the pedal is fully retracted. There should also be adequate pedal travel to fully compress the master cylinder. Pedal return springs and stops are highly recommended.

http://www.wilwood.com/PDF/ds260.pdf

[Damn True]

I'm thinking pad knockback.

What spindles are you running?

[6'9"Witha69]

What spindles are you running?

One off.

http://www.iimuchfabrication.com/New.../IMG_5877.html

[David Pozzi]

I question the lifting the pedal with left foot. You should have a return spring if that helped. I have seen some master cyls with extra friction, especially the 5/8" bores, they don't return easily when bleeding the system.

If it's pad knockback. Add 2 psi residual pressure valves to your lines. I'd do both front and rear lines.

Pad knockback would happen right away, as soon as you hit some bumps or a very hard corner. If it's fluid heat up, it would take a few laps, which is what you have reported. I'd convert to high temp fresh fluid and consider adding the residual pressure valves if you still have trouble.

I would bleed the brakes and get fresh fluiid in the system just prior to each track event you run.

Pegasus racing sells stickers you can put on the calipers to record temperatures reached.
David

[parsonsj]

Good feedback guys. I appreciate it. I'm using a balance bar, so that could be the cause of some extra friction that keeps the pedal down a bit. I did notice when the pedal was down that isn't any line pressure. I would pull the pedal back up with my left foot, and the next brake effort would feel very nice. Then the pedal would go down after a few more turns ... repeat, etc.

I'll have the wheel bearing out of the car tonight/tomorrow (it's a C4 bearing).

Adding a return spring is an easy addition. I'll have a look at that too, since one other problem is that my brake lights are a bit sensitive.

jp

[TitoJones]

John-

Check your wheel bearings first and make sure they don't have excessive play; then flush all the fluid and replace it with the Castrol race stuff that comes in the metal can. After that call Wilwood and see if they offer some knock back springs to put behind the pistons to prevent this on the track.

Tyler

[parsonsj]

Well, that'll explain it: both wheel bearings are worn out. On the right side, the wheel had 1/4" movement at the rim, and the left side almost as much.

When I put the car together, I wasn't willing to pay $750 for a pair of bearings ... and I found these at 1/3 of the price. On eBay.

I'll bet I bought some painted previously failed bearings. I guess one gets what one pays for.

Now where is Frank and his Chevy account?

jp

[parsonsj]

Wait, there's more! It turns out that C4s and C5s are plagued by wheel bearing problems, especially those that see track time. Even the factory teams had/have problems. Bopping around 'Vette fora has taught me that most track guys consider the wheel bearings wear items along the same lines as brake pads.

Anyway, I've found that Timken 513009 bearing/hubs will work fine, and cost about 1/4 as much as GM dealership items, and are probably made in the same factory. 2 are on the way.

And, my brake pedal has a return spring now, so that resolves the brake light problem.

Now for the leak in the ps cylinder. Tracks are hell on cars!

jp

[6'9"Witha69]

most track guys consider the wheel bearings wear items along the same lines as brake pads.

Tracks are hell on cars!

Yes, yes they are.


Are you still going to flush the front system?

[MonzaRacer]

Whoa wait a minute, Dot % is silicone just use a good brand of dot 4 with a boiling point over 500 degrees.
Also i like to flush with a Phoneix Injector and reverse flush/bleed because if there is any air then it usually comes to the top.
I agree on the 2 PIS check valves for positive pedal performance and a simple return spring is mandatory in my opinion.
Also a tTHIN coat of silicone or moly brake lube on the backs of the pads and and on all sliding points.
Just dont use enough to drip off at high heat. This practice will aleviate some unwanted squeaks and groans.
also Ican source a product that is applied to new or roughed up pads AND brake shoes that will help in the quelling of any squeaks attributed to harder performance pads.
Never autocross or do any spirtited driving with silicone brake fluid, that stuff is dangerous.
A friend of mine bought a car and barely stopped at drag strip, stupid builder used silicone Dot 5 brake fluid,,, it wont buckle paint but barely stops either.
Good luck.
Lee

[parsonsj]

No worries. I'm well aware of DOT brake numbers and ratings. No silicone for me ...

Yes, I'm going to replace the front (and back) brake fluid, but I'll wait until a week or so before the next track day (August). Street driving will be just fine, I'll bet. I'll get some of Wilwood's 570 degree stuff for that. I use a Motive Power bleeder, so it's pretty simple to replace the fluid.

As far as sliding joints go ... I don't got none. Fixed calipers all the way around.

What's the thought on residual pressure valves? Absolutely required? Or maybe I see if new fluid, a return spring, and good wheel bearings fix me up?

jp

[2yellow69]

Here's some helpful info on brake fluid.
Matt
http://www.afcoracing.com/tech_pages/fluid.shtml
I don't know how to do a link so copy and paste!
Here's another one.
http://www.motorcycleproject.com/mot...rakefluid.html

[parsonsj]

Well, the wheel hubs have been replaced. Both were toast: the driver's side can't be turned by hand, and the passenger side has a click/grind spot.

I've put 900 miles plus two track events on the car. I sure hope the new ones last longer. :(

jp

[MonzaRacer]

Hey big trick is torque specs for the bearings.
when I worked at Firstone we had many of the newer GM/Chryco cars come in with quickie shop(ie Midas, Meineke,etc) installing new ones during alignments and they never used proper torque specs use the ones rated for the bearing application(ie vette for vette, blazer for balzer etc).
Good luck

[chicane67]

Just as an FYI point... this is on brake fluid and performance use:

http://www.stoptech.com/tech_info/wp...fluid_1a.shtml