NTMD8R

I have built my '94 Z28 into a road race car.
Yeah... I know... this is the Corvetteforum.... I have a 2K convert that I have road raced, and a '67... been vetting for over 20 years.

But I have built the Camaro for road-racing, just so I won't cry if I should have a major boo-boo (you know.... hit the wall, or have some other driver make a tragic mistake) .


Anyway.... We left the stock LT1, 6-spd, and 3.42 gears in the Camaro.
Put on headers and 3 inch exhaust, and K&N open air cleaner.
Installed 13" ZR1 rotors and Wilwood calipers on the front, stock 12" rotors and calipers on the rear.
Use Wilwood black pads on the front, Hawk HP+ on the rear, with Motul 600 synthetic fluid.

When we first built the car, the brakes were good... hard pedal about 2/3 way down.

Now, after 4 or 5 events, I can't get a hard pedal no matter what we do.
Rotors are still great, pads are still more than 60% left, no leaks whatsoever, bled the system until we are nuts. Changed the fluid completely... no difference. Installed a new master cylinder... no difference.

Still get a spongy pedal... the brakes work... but pedal is very spongy... and I can actually push down to the floor if I push hard enough. I couldn't do this when they were originally installed.

We are going nuts trying to figure out what we can try next.

Any ideas????? Anyone?????

no cure

Gary,
is this an ABS car? did you install a rear bias proportioning valve?

the first 5 events it worked totally normal?

(hairline) cracked brake line? (very slightly) unseated banjo fitting?

you've examined the calipers (no leakage around pistons?)

gotta be frustrating. does she still stop hard even with sqishy pedal?

David

Corvette55

I have had the same problem twice on my 93 vette. I dont know if one of these are your problems but it may give you some places to look.

First the return line from the ABS unit to the Master Cylinder was clogged trapping air in the abs unit. I used a vacuume pump to clear the line and it was fine after that.

The second time I had the problem a few years later one of the inbound front brake pads was jammed and not floating in the caliper. When you stepped on the brakes the piston would actually bend the pad. This allowed me to see that there was pad left (without removing the caliper) and blead the brakes without fixing the problem. The pedal was hard when there was no power assist and soft with power assist because the pistons would actually bend the pad until it reached the rotor. I replaced the pads, and use brake caliper grease on all the floating parts before every track event.

NTMD8R

David... Hi... haven't seen you for a while. As you can see, I am moving up a bit. But I still do the PDC stuff.

Yes it has ABS, but I don't think it is functioning.... I can lock them up sometimes. The car was an ICBC write-off/re-build, so I don't really know what was actually good/working. It has a proportioning valve, and I installed a DRM bias spring.

Another thing I forgot to mention is that we also installed SS brake lines.

Actually, I noticed that at each event, they seemed to be getting spongier.... more so at each event...but they still work AWESOMELY. I can still outbrake most cars in all the turns. At least, they did until last event.

I have an event this coming weekend, and I am somewhat apprehensive about the brakes now.

Corvette55:.... thanks for the tips. I'll see what we can do to check the ABS lines, and to ensure the pads are not "sticking".

One other thing I have just come across is "Pad Taper". It seems that some pads, when used aggressively, can taper from the inner circumference to the outer, due to different circumferences of the portions of the rotor where the pad touches.... usually wearing more to the outside of the pad as opposed to the inside. This can cause the pad to wear "tapered", which can cause the pedal to feel spongy. If I consider how I felt they were getting spongier each time I used the car, this might seem to make sense.

So... I am going to check into this also.

But..... any more info/help is most certainly appreciated.

corvette dave

Gary,
Pad taper is a huge problem.
Put on a new or slightly used (non-track) set of front pads.
That will probably give you the answer.
Dave

SWCDuke

Yeah, we had a discussion on this a few weeks ago. I think pad taper is the issue. On a street car that you track the pad taper goes away after a couple of hundred miles of normal street driving, so the loss of pedal height and sponginess goes away. The taper can either be ID/OD or leading edge/trailing edge.

One thing you might try is to swap pads side to side, If the inside and outside pads are the same you can just swap them side to side on the caliper. If the pads are inside/outside specific you can swap them side to side on the car - LF to RF and vice versa. Either way this will run the pads "backwards", which will take out leading edge/trailing edge taper, but eventually taper them in the opposite direction, so you just swap as often as necessary to maintain good pedal height. They'll need a quick breakin after every swap, but I think that might solve or at least mitigate your problem. You should probably swap the pads side to side after every event or even during the day if the pedal goes away. The only other solution would be to install new pads when the pedal height drops.

Assuming pad taper is your problem you can probably actually measure it.

Duke

P.S. I recently tried this on my Cosworth Vega. The pedal was not that bad, but I swapped them (side to side on the car since the pads are inside/outside specific) and after about 50 miles of street driving, the pedal felt firmer.







[Modified by SWCDuke, 11:04 PM 8/31/2002]

Richin Chicago

Sounds like pad taper to me also. Did you use any break in proceedure for the new pads? I take my car out after a new pad install and make 6 hard stops from 45 MPH, but space the stops out so the brakes never get hot. I then do it again from 60 but, again, never let the brakes get hot. For some reason pad taper is much worse if the pads aren't bedded in. My guess would be that the limited contact of a new pad wears it in a bad way if you start heating them up without bedding them. Also pads that aren't good at high temperature, obviuosly, makes this worse.



[Modified by Richin Chicago, 8:50 AM 9/1/2002]

h rocks

Your calipers are fried. All of the factory slide type calipers will give you pad taper, and when it's really bad, (as your sound) you have gotten them pretty hot. Especially when you start utilizing real race pads, the life expectancy of the factory parts diminishes rapidly. The calipers are spread out and the leading edge of your pads are really taking a beating. But that's life at the track.
I would not waste the effort trying to save some $$ flipping the pads to "even out" the taper. First, it's not safe, secondly, you are going to hyper extend one piston, and knock back the other. (that's full of track dirt and pad compound) Stock type 12" rotors an factory calipers are not going to cut it, no matter what you do. sounds like you're beyond the capabilites of factory components. :)

silverz06vette

I may have the same problem on my Z06. I ran 2 track days back in June with stock fluid and pads. I beat the brakes up pretty good. Pedal was very soft after the event. My pads where somewhat tapered, so I sanded them down to even accross all 4 front pads. I even tried to switch pads front left to right side. I flushed out the fluid with some better stuff and installed carbotech panther + pads on the front. The pedal felt better but still did not have the full pedal feel to it. Still soft but not as bad. I ran the car again at the track for 1 day a month ago, brakes worked good. But I still don't have full pedal travel.

Is is the heat that kills the calipers and causes them to "bend"? Or is it repeated hard brake application? Would keeping new stock calipers cooler help with some additional brake ducting?

Richin Chicago

I fried a couple of sets of caliper boots and wrecked a set of calipers before it dawned on me that I needed more cooling. I installed the Doug Rippie Front Brake Ducts and haven't had a problem since. I have repeated this so many times I sound like I am on a crusade but IMO it is the best $200.00 you can spend if you are going to run on the track. Also I see better stopping and much increased pad life. A final benefit and, again, just my opinion.
Some brake pads get nasty after you take them past their heat designs and will score your rotors. I had to find this all out the hard way!
http://www.dougrippie.com/drm/whats_new.htm

I have had quite a bit of track time this year and everything works fine, from stock rotors through stock calipers.

NTMD8R

Wow.... thanks for all the responses guys.
Just to give a bit more info.... we built this car this year, and have only driven it 2 race weekends, and 2 "hot lap" days, so I would not have expected the calipers to be toast already.

HRocks.... I have the Wilwood Superlite II calipers on the front. Yes, I expect they have gotten quite hot, for I do brake aggressively.

Richin Chicago.... I have 3" cooling ducts installed from the front foglight openings to the inner fender liners.... stopping about a foot from the rotors/calipers.

Well, I can't get new pads in time for next weekend, so I plan on trying to swap the pads to see if things improve. When I get the new pads (next week.... time....that's a problem with being in Canada and ordering from the US ), I will install them, and see if things are better. If not, then I'll have to see about servicing/replacing the calipers.

Thanks again everyone.

TeamZ06

Just out of curiosity, did you try contacting Wilwood and get their opinion/suggestions?

David Lively

If you're running 6-piston non-floating calipers then pad taper is almost certainly NOT your problem. My understanding is that taper is a side effect of caliper flex and the design of the stock GM sliding caliper; a non-sliding type shouldn't have that problem.

Of course, it's easy to tell. Take your pads off and look at 'em. If they're tapered, you'll be able to see it easily.


[Modified by David Lively, 10:45 AM 9/3/2002]

NTMD8R

Haven't called Wilwood... perhaps I should just to see if they can offer any suggestions.

The fronts are Wilwood.... the rears are stock GM calipers.
Maybe it's just the rears that are creating this problem for me?

David Lively

Probably not. As you're no doubt aware, the front wheels perform the vast majority of braking. You may be seeing pad taper on the rears (admittedly possible, though I've never seen it on my car) but I don't see the rears would significantly affect pedal feedback in this manner.

TeamZ06

this is a bit of an off-the-wall wild-behinded approach, but may be worth considering if all else fails. The reality is that in order for the pedal to go all the way to the floor you have to either have bleed-by (which you pretty much eliminated most components with parts swapping), external leakage (eliminated by visual inspection) or a substantial amount of fluid is flowing into some part(s) of the hydraulic system. IMO it would take an extremely severe case of pad taper in order for this to occur, which should be easily detected visually. You don't have leakage, we'll assume no bleed-by in any of the major components, so that leaves fluid flowing into some part of the system. The stainless flex hoses pretty much eliminates any expansion there (with the rest of the system essentially hard piped) and if a visual inspection doesn't indicate the world's worst case ever of pad taper then that only leaves the following in my mind.

ABS braking systems have secondary circuits (accumulators, flow valves, etc.) which are usually closed during regular braking and bleeding operations. There's a possibility that you have either malfunctioning components or have somehow trapped air in the secondary ABS hydraulic control system. There should be an automated bleed procedure that can be activated with a scan tool that will purge air from secondary system if that is the case, or will abort if some other type of malfunction exists. This seems logical in my mind since you did notice that the ABS system does not appear to be functioning properly.

good luck,

Mark


[Modified by TeamZ06, 7:25 PM 9/3/2002]

h rocks

NTMD8R, the Superlites are a small pad caliper typically used in rear applications. (there are WW 6 piston versions that use the same pad as the Superlites) Do you have the calipers properly shimmed (+- .005") so that the rotor is perfectly centered? Are ;you still running the stock (little PBR) rear calipers? Once you improve the fronts, you can then add more rear brake. It makes a huge difference in overall braking. The fronts will still do most of the work, but you will notice that the car really hunkers down instead of dives when you lean heavily on the binders.

NTMD8R

OK.... here is what we have found out, and done.

I "bit the bullet" and decided to take things apart.
Looked at the rears first.... wow... pad taper very visible.
Funny thing... the inner pads were tapered longitudinally, and the outer pads were tapered laterally.
I was also concerned about the calipers, so I replaced them, and the pads, and surfaced the rotors.

Not much improvement.

So I pulled the front pads. WOW!!!!!!!! What a surprise.
Just like the rears, the inner pads were tapered longitudinally, and the outer pads were tapered laterally.
Now this taper is not microscopic... it is VERY visible to the naked eye.
I did not have any spare front pads (Wilwood) available, so I merely swapped inner for outer on both wheels.
This has made an improvement. Enough so that I feel comfortable to race the car this weekend.

I have a friend who is very technically knowledgeable, and he explained to me just how this pad taper happens.
The caliper is "U" shaped, and the inner leg of the "U" is where the caliper is fastened to the hub/spindle assembly. Therefore, this inner leg is about as rigid as it can be. The inner pad therefore cannot move around, and the braking forces are applied to the pad very equally. Except that the leading edge of the pad experiences most of the work due to it creating heated gases which follow the rotor to the rear of the pad, causing it to not make quite so much contact/pressure as the leading edge. This results in longitudinal taper.

However, the outer leg of the "U" is not affixed nearly as rigidly as the inner. As force is appled (quite seriously), the outer leg actually flexes, and the legs of the "U" become not parallel... the outer leg actually spreads apart a bit, thereby causing the pad to not be parallel to the rotor. This results in lateral taper.

I have ordered 2 sets of Wilwood pads, and will replace them after this weekend event.

Wow..... watch out for pad taper.

HRocks... I did not know that the Superlites are for rears... these are 4 piston setups. I do know they have 6 piston setups.... but my budget could not afford the $2K for them.
And I am still running the stock rears.

This is my first (novice) year, so I took it easy with the initial build, moneywise. Over the winter we will definitely do some upgrades.

Thanks everyone for all your help and suggestions.


WATCH OUT FOR PAD TAPER !!!!!!!!!!!!!!!!!!!