CStewTAMU

I have a 1994 Corvette with the stock 12" brakes. My car is an automatic, so I can't really engine brake by downshifting to save wear/tear on the brakes.

I use the Hawk HP+ autox pad in the fronts when racing with 275/40/17 Kumho Ecsta V710s on all 4 corners.

Last time I went autocrossing, I must have boiled my brake fluid. It was a 7 event autoX (14 total laps) and I bet after 3-4 laps, my brakes became VERY spongy, and the brake light was coming on by the end of the day.

I bleed my brakes quite frequently (about 5 times a year, and usually before racing).

However, my brake fluid was somewhat fresh and clean (maybe 2-3 months of street driving with the only hard braking every done being the heat cycling of the tires which was a few hard stops).

I am guessing that I must have boiled the brake fluid and the air bubbled in the reservoir must have sucked air into the system making the pedal very soft.

Ive been told upgrading to the 13" brakes really won't help much. It won't make you stop faster. It would only help with brake fade which I would think is more pad/rotor related-not really brake fluid.
-I am satisfied with how fast the car stops with good fluid. I just wish I could keep it from boiling.

I have been using the Castrol synthetic that you can pick up from from any auto zone, OReily's, Pep Boys, etc. as I think it has a higher boiling point than the others on the shelf.

How do all the other forms of racing keep brake fluid temps under control?

froggy47

There are MANY higher temp rated fluids. Lately I use Wilwood 570. Good bleed & fresh fluid as needed depending on use. Also ATe Super Blue is another I use.

CStewTAMU

Can you mix brake fluids? I usually think of changing brake fluid like changing transmission fluid...you really just dilute the bad stiff with fresh fluid until is as much new fluid as you can get it.

I drive the car regularly. It's not my true dailey driver, but it gets plenty of street use.

Are there any drawbacks of using these racing brake fluids?

Bill Dearborn

Are you sure you are having a problem with the brake fluid? What kind of an autocross course were you running? Length? Speeds? Number of Turns? I have and have seen many people run long low speed autocrosses with years old brake fluid and not have a problem.

Bill

froggy47

If you do a complete bleed (fresh fluid in the brake master cyl.) you are not mixing (a tiny amount in the abs module is not replaced unless you do a tech 2 service)

But the answer is yes you can mix fluids - but it's better to do a complete service. Exception is silicone brake fluid - do not use this.

BTW if the brake light went on & off you probably have LOW fluid in the master cylinder.

You may also have an issue with the power brake booster, if the fresh fluid & bleed do not help I would look there next. Or you may want to check the hoses that run to the calipers, they can go bad even though the LOOK fine on the outside, are they original? How many miles?

63Corvette

I recommend Castrol SRF..............much more expensive, but worth every penny!

redazz

+1 for srf more money but worth it high boiling point dry and wet.
john

Aardwolf

What hot temp is the fluid you're using? There are options: higher temp fluid, heat shims, stainless pistons, air ducts.

Sidney004

It may also be related to the pads. I have no experience with the HP+ but some pads(and worn ones) have a high thermal conductivity and transfer a lot of heat. I also find it hard to believe that a few autocrossing runs are boiling your fluid. You may have also spread your calipers, are your pads showing any tapering? If so, post the pics.

CStewTAMU

Lots of questions to answer. I know the basics of brakes, but forgive me if I have to ask questions.

To a previous poster: When the brake light came on late in the day, it was when the pedal was REALLY spongy. The light would come on when I pressed the brake. If I tapped the brake again it would go off. However, as soon as I applied the brake again, it would come on again. The car would stop fine, but I would be in trouble if I needed a panic stop. The reservoir was full, and the flulid looked clear-not brown as the fluid was only 2-3 months old. A few days after the autoX, I bled the system and that fixed the brake light problem. Also, the pedal is firm again. Brakes just fine.

To previous poster: What do you mean when you say tapering? Is that synonomous with the pads wearing evenly? The pads don't look like a "shim" if that's what your getting at.

My rotors are OEM, and pretty new. They have at most 2K miles on them, mostly street driving.

My brake hoses are aftermarket stainless steel braided. They also fairly new-at most a year.

I visually inspected everything when I removed the autoX tires after autoXing to put my street tires back on and didn't find any leaks.

I agree with another previous poster that I couldn't believe that my brakes basically failed after 3-4 laps. However, I do need to reiterate that I have an automatic, so I can't really engine brake or downshift to take some of the load off the brakes. When stopping in a hurry, all the load is on my brakes. I kinda joked with my autoX buddies that day that, my lap times actually got better with worse brakes as it forced me to be smoother and take better angles. I think the simpel advice of "use your brakes as little as possible" isn't a bad tip to newcomers in autocrossing. However, everyone is a little different. Two of the FTD's took the same turns very differently. I was a corner worker and got to observe quite a bit.

I would say the course was right at 1 minute long. The FTD was probably a 57 and my fastest lap was maybe a 61.8 or so. My car obviously much slower than most. Ran 14 total laps in sets of 2 with about 30 min down time between each set. I have no brake ducts, but I might consider that. I would say the highest speed reached might have been 65mph, although I didn't really have time to look.. However, that was only in one stretch with the rest of the course more tight (under 40mph). It was a NCCC event, and we run our courses pretty open. I would say there might have been 10 turns, all more than 90 degrees with some being over 180 degrees where we would back track over where we just had been.

Another poster made a good point about how the Hawk HP+ AutoX pads may absorbe heat much more and that could by my problem. This was the first time I had autoXed with them. Before that, I was just using the autozone ceramics which are fine for street use.

froggy47

It's not the pads, I have used HP+ on two Vettes for autox and track days & they are a very good pad for those applications.

I think you maybe had air in the system & got rid of it.

I would still switch to a higher temp fluid & complete bleed as needed.

At track days many drivers will bleed before the event, and bleed again at the noon break.

Adam@Amp'dAutosport.com

1lapRacer

My 91 had a similar problem that after I bled them the pedal would get soft. This was after I had switched out a caliper. To finally resolve the problem I had to bench bleed the master cylinder and then get the abs unit to run through a few cycles by locking up the wheels at low speeds. I know of at least one other C4 autoxer that had an issue with sucking air into an abs unit. Switching to good fluid is a must as others have stated.

Bill Dearborn

When I ran my C4 I never had a pad tapering issue that caused a long pedal. On C5's I had it all the time with the stock calipers.

There is no way you generated enough heat to boil the fluid running a one minute course even if you ran two laps. Autocrossing just isn't that hard on the brakes. HP+ pads work well in an autocross so I doubt they are your problem. As long as they were over 1/8 thick they should have been fine.

You are no worse off with an automatic. When you stop hard with a manual transmission car the brakes do all the work. In fact the engine could add brake load if the driver hits the brake and gas pedal at the same time.

Brake ducts will not help as you aren't going fast enough through most of a course like that for them to provide much air.

Do the low cost things when looking for a solution but I suspect you may have a mechanical problem (master cylinder) Vs a pad or fluid temperature problem.

Bill

jwt1603

I chased the same problem in my 94 for a year. Exact same issues. I would bleed the brakes and after 3 to 6 passes the brakes would get spongy and the light would come on. Pedal to the floor after about 9 passes. I use Motul fluid and I use either the Hawk HP+ or the Carbotech auto-x pads. I would get air in the rear calipers only. A fairly big pocket of air out of each side. After bleeding they would be fine for another 6 passes then go away again.

I replaced the front and rear calipers. (Fronts w/ C5 brakes). Then I replaced the booster and the master. Then I started using a pressure bleeder. Still would get air out of the rear calipers. Then one day when I was bleeding the brakes I noticed that the rear bleeder screws didn't feel right. I would crack them a quarter turn to bleed and they wobbled in the caliper, I replaced the bleeder screws and that didn't help. I recently replaced the rear calipers again and on these the bleeder screws are very snug in the caliper when backing them out to bleed.

Since replacing the rear calipers for the second time earlier this year we've run several NCCC two day event weekends with two drivers, that's about 50+ runs on the car each weekend, (don't ask about my tire expenses and how many Hoosier A6s we go through every year) and I haven't had the problem again. I do bleed between weekends but I don't get any air now. Brakes are good and firm from the first pass Saturday morning until we put it back in the trailer Sunday afternoon.

Might not be the same problem with yours but I would suggest checking the bleeder screws and their threads. My theory is that when they rebuilt the calipers they ran a thread chaser in the bleeder port and opened up the threads just enough that when the pressure was released they would suck air in. They didn't leak fluid out because the pressure tightened the thread seating surfaces.

Just my theory but the only thing different now is tight bleeder screws and I don't have the problem anymore.

froggy47

Mt friend (T1 racer) was getting similar symptoms. Soft pedal, bleed fixes, run car, soft pedal, bleed fixes, etc.

Turns out air was entering via the abs module. The bleed does not involve the abs (unless you do tech 2 bleed).

So whenever he raced, the first few abs activations let the air (in the abs) into the rest of the hydralic system & pedal went soft.

He put a new abs on & problem fixed.

There are a LOT of connections on an abs.

So check the abs for air leaks.

fwiw.

RX7 KLR

Are all of your hubs good? Excess play in the bearings will knock the pads back and make the pedal feel long.

vettehardt

My dad has a 90 ZR-1. We run he Valvoline DOT 3-4 syntetic and the HP+ pads. We have not had a problem with braking with this setup. As far as engine braking goes, we usually shift to 2nd gear as soon as we can and never downshift unless it is a very, very slow corner. So engine braking never comes into play.

We did have a problem at one time. My dad bought the Performance friction pads that had the lifetme warranty. He wore down those pads and got a replacement set. Ever since we put those pads on, we had a soft pedal (but a light never came on). We bleed, and bleed, and bleed. We used a power bleeder and even used a Tech 2 to cycle the ABS. Nothing helped. Dad got to talking to someone who told him the replacemnt pad were junk and caused soft pedals. We changed out pads and haven't had a problem since.

As far as your problem, Sorry I don't know what it is causing it. With the light coming on, it sounds like a fluid level problem or a pressure problem. You don't have any leaks anywhere? It may be leaking internally inside the booster.