Road racing: Q on brakes

[Slow old poop]

I have a 90 Eclipse GSX with StopTech brakes, running 21 psi boost.
After every blast down a straight under boost, my pedal goes soft, and needs to be pumped twice to get hard. It feels exactly like I boiled the fluid, but that's not the problem. Two weeks ago, it did the same thing in the rain at MidAmerica. Once it's up, I have plenty of brake, and the brakes don't fade, but it cuts a couple of seconds off my lap times with all the pumping.
We have flushed the system and replaced it with Motul.
Pads are good, with new rotors.
Thinking that maybe the big boost was overcoming the stock check valve, we added a crank vent check valve to the system.
We ran it again at Gingerman this weekend, and had the exact same problem.
Any other road racers have this problem?

Rich/slow old poop

 

[Greg Collier]

Rich, I've had nothing but success with my Stoptechs. It sounds like you've done everything right, and I'm sure the system has been bled properly. Let me give you the e-mail of Matt Weiss over at Stoptech, he's they're main engineer and was critical in the design of their brake package for our cars, [email protected]. Matt's a really good guy and will figure out what you need.

Greg

 

[terefic181]

These are from the Mitsu tech manual:

BRAKE BOOSTER OPERATING lNSPECTION
For simple checking of brake booster operation, carry out the
following tests.
(1) Run the engine for one or two minutes, and then stop it.
(2) Step on the brake pedal several times with normal
pressure.
If the pedal depressed fully the first time. but gradually
becomes higher when depressed succeeding times, the
booster is operating properly.
If the pedal height remains unchanged, the booster is
faulty.
(3) With the engine stopped, step on the brake pedal several
times with the same pressure to make sure that the pedal
height will not change.
Then step on the brake pedal and start the engine.
If the pedal moves downward slightly, the booster is in
good condition. If there is no change, the booster is faulty.
(4) With the engine running, step on the brake pedal and then
stop the engine.
Hold the pedal depressed for 30 seconds. If the pedal
height does not change, the booster is in good condition.
If the pedal rises, the booster is faulty.

If the above three tests are okay, the booster performance can
be determined as good.
If one of the above three tests is not okay at last, the check
valve, vacuum hose, or booster will be faulty.


Caution
The check valve is press-fit inside the vacuum hose and
do not remove the check valve from the vacuum hose.

Caution
If the check valve is defective, replace it as an assembly
unit together with the vacuum hose.

Don't know if this will help you out, but good luck!!!



Tom

 

[Slow old poop]

Thanks for the reply. I sent an email to Stoptech.
Are you running power brakes? We were thinking about maybe going to non-power brakes to get rid of the vacuum problem.
Rich

 

[Slow old poop]

These are from the Mitsu tech manual:

BRAKE BOOSTER OPERATING lNSPECTION
For simple checking of brake booster operation, carry out the
following tests.

Tom: Thanks. I can't wait to run those tests. Too bad it's so late tonight, or I'd do it now. I can't go start the mutha now, because it is midnight and it's a very LOUD car!.

Rich

 

[terefic181]

Yes, I'm still on the stock system (Power). So you think your having problems do to the boost your running, bleeding over into the booster?

I'm sure Matt with StopTech can get you on the right track. I just thought you might want to see the Mitsu tech on it.

Do you have access to the pedal adjustment specs?

http://www.dsmtuners.com/forums/showthread.php?t=184162



Tom

Edit: Added URL

 

[Slow old poop]

Yes, I'm still on the stock system (Power). So you think your having problems do to the boost your running, bleeding over into the booster?

Well, that's what we thought initially. We put in a crank vent to get a more industrial check valve in there, but that didn't cure it.

>Do you have access to the pedal adjustment specs?
http://www.dsmtuners.com/forums/showthread.php?t=184162

Thanks. I'll try that, too. The problem is, when I hit the brakes the first time, the pedal goes down, with no braking. Two pumps, and the pedal is high and firm.

Maybe it is the booster. The car is pretty old.


Rich

 

[Dauntless]

Rich,

My experience has been that bad boosters give very hard pedal, not an initially soft one. That's because the booster is designed to reduce the effort required to get full braking. When it goes bad it is no longer helping you so the pedal seems very firm from the outset.

OTOH, assuming no air in the system, an initially soft pedal makes me suspect that your master cylinder may be bypassing internally and needs to be rebuilt or replaced.

The other issue I'd look at is rebuilding your calipers. They tend to gum up under road racing conditions due to repeatedly heating the fluid to its thermal limit. Combined with the tendency of race pads to experience knock-back (where they don't return to close proximity to floating rotors), you may have to pump the pedal before each brake application. I can't tell you how many times I have solved seemingly intractable braking issues by rebuilding the calipers (disassembling, cleaning and replacing all the o-rings).

If you've thoroughly bled the system and still have the same problem, then suspect one of these two issues.

Good luck, and glad to have another road racer here!

Stan

 

[Slow old poop]

Stan:

Thanks for the advice. I doubt that the calipers are bad, because the previous owners never really used the brakes hard. They look pristine, almost new. In fact, the previous owners never really ran the car very hard. We are finding things that any serious road racer would have found ages ago, such as boost leaks and broken brackets.

You seem to have eliminated the brake booster as a cause. My symptoms are definitely not a hard pedal.

I like your ideas on the master cylinder, though. The car is pretty old, and I bet it was never serviced.

Looks like I have a lot of stuff to look at. This may take awhile to cure.

Rich

 

[Slow old poop]

After considering what Stan said (above), and after reading the tech article on knockback on the StopTech site, I think maybe knockback is my problem. In a nutshell, it means that hard cornering is distorting my wheels, rotors and whatnot, forcing one side of the pistons away from the rotor. Then, when approaching a corner, it takes two pumps to get the pistons back into position.

The previous owner replaced the left side hub and wheel bearing, and said it helped the braking problem, so I think I'll replace the right side hub and wheel bearing.

Qs for the group:

1. What's the best hub? Stock Mitsubishi? Somebody else? Is there a racing hub?
2. Should I avoid aftermarket suppliers?
3. Is there a 3000GT/Stealth hub that would work? The Eclipse hubs are about half the size of 3000GT hubs, and I figger that bigger is stronger....any opinions here?
4. Is there some way to strengthen the hub, like cyrotreating or heat treating?
5. Do you know any way of shimming or strengthening the wheel bearing?

Rich

 

[Greg Collier]

Rich

My hubs are as old as my 90 Laser with 100000K on them and I'd have to assume that my cornering speeds are equal if not greater than yours. What type of pads are you using? I've been using the Porterfield Club Racing Pads because I got a half dozen sets for free. I will be changing to the Hawk Blue's although because they're a better pad for my application. The Axxis pads that come with Stoptechs will work on the street but for balls out breaking at high speeds they'll disintegrate. Just a thought...

Greg

 

[Slow old poop]

Rich

What type of pads are you using? I've been using the Porterfield Club Racing Pads because I got a half dozen sets for free. I will be changing to the Hawk Blue's although because they're a better pad for my application. The Axxis pads that come with Stoptechs will work on the street but for balls out breaking at high speeds they'll disintegrate. Just a thought...

Greg

I have Cobalt Friction Spec B up front and Axxis on the back.
No complaints about the Cobalts, but I haven't really hammered them yet; that is, I haven't been to Road America with these pads yet, where it hauls down from 140 to 75 for Turn 5. That's the killer test for pads.

I think I figured out how to add photos to my post. They are low res, but I hope you like the photos of my car

Rich