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Discussion Starter #1
so im not really sure how to explain this but ill try my best.
i have 4 wheel willwood disc brakes with a factory booster and a z28 fat boy master cyl. the check valve was replaced this past winter.
i notice no problems at all under normal driving. when i go to the auto x events and push the car fairly hard i still have brakes but the pedal is down about half of its travel.
my motor makes about 15 psi at idle and i cant remember where it goes from there.
again i have brakes but half of the travel. any ideas as to whats going on? i am suspecting its something to do with the booster or vacum but i dont know.

thanks in advance for the help.
 

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Do the brakes still function properly or is there a loss of any braking at all? If they start to fade and get hot, the fluid if old can boil which results in the pedal getting spongy which will take more pedal pressure to compress. A flush and fresh fluid can help. Also DOT-4 has a higher boiling point than DOT-3 so that can help. Usually fading with good fluid will still give a hard pedal but greatly reduced braking. Also old rubber lines can start to swell and require more pedal travel.

You can check the booster by shutting the car off then pumping the brakes several times to bleed the booster down. Then hold the pedal down and start the car. The pedal should rise against your foot.

I have had a check valve be bad right out of the box before. The car stopped fine in my case but something didn't feel right. You can check it by just pulling it out and blowing in it. You should only be able to blow through the side that plugs into the booster, not the side that connects to manifold vacuum.

It's possible that you're just not able to rebuild booster pressure fast enough between applications when you're autocrossing but it sounds like a fluid issue. Usually no vacuum manifests itself in a hard pedal.

Do you have good stainless hardlines, braided flex lines, fresh pads, and fresh fluid?
 

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C-clip axles and fixed calipers...

I'm in the process of putting together a new rear and having axles made. The guy akked me if I was running Wilwoods in the rear, and if so he'd make the axles with an extra tight button, just for that issue. Since I'm using a floating caliper (and don't auto-cross) he said no worries.
 

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Check end play on your rear axles. Posi-traction rears have much less play. Open rears are terrible. GM made several thickness C clips to help reduce play, you can also place shims behind the C clips to reduce play.
David
 

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C-clip axles and fixed calipers...

I'm in the process of putting together a new rear and having axles made. The guy akked me if I was running Wilwoods in the rear, and if so he'd make the axles with an extra tight button, just for that issue. Since I'm using a floating caliper (and don't auto-cross) he said no worries.
Al, what are you referring to with the button? The bearing retainer?
 

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Todd -

The "button" is the part of the axle at the very end between the c-clip groove and the cross shaft in the carrier. There is a little "slop" built in, which results in the end play on c-clip axles. If you make the button a tad longer, the cross shaft forces the axles out as far as they'll go and there will be less end play. Less endplay = less pad knockback. That's how the axle guy explained it to me.
 

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are all 4 cross drilled and slotted dics's
and are you running dot 5 brake fliud?
by what your describing, it doesnt sound like brake fade at all?
how is your pedal when the brakes are cool when your done racing? they are fine right?
 

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Gotcha Al. I have bolt in tapered bearing 33 spline axles with no end play felt and still get it. The only way to eliminate it is with a floating caliper, rotor, or axle IMO. Systems with a larger bore master cylinder cover it up in most instances due to the extra piston volume. I like the feel of manual brakes on teh race track but it has compromises and knock back is one of them. I'm currently getting ready to swap to a 1" bore to try it out. It tough pumping brakes in autocross! :D
 

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Have you considered a hydroboost and an even bigger MC, like 1 1/8? It wouldn't change the knockback at all, but should have even less effect on pedal travel. Certainly not the feel of manual brakes, though.

Somewhere something is moving around. It's hard for me to imagine a cast iron rotor flexing w/o breaking. I guess any flex at the hub (front or rear) would be multiplied quite a bit with a 12" or 13" (or larger) rotor between it and the caliper.
 

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Discussion Starter #11
you guys have it figured out. i never thought of the movement in the axles until you mentioned it. that has to be what it is.
everyting is new in my braking sytem this spring.

the pedal is fine hot or cold on the road. but on the track with a couple of cones to go around and the pads are pushed out. i just have to get used to the longer stroke of the pedal. i have brakes but im near the end of the stroke.

im sure thats what it is. maybe ill try to design a "floating setup for them" or just wait until i blow my 10 bolt out and swap in a nice 12 with the eliminators.
 

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Have you considered a hydroboost and an even bigger MC, like 1 1/8? It wouldn't change the knockback at all, but should have even less effect on pedal travel. Certainly not the feel of manual brakes, though.

Somewhere something is moving around. It's hard for me to imagine a cast iron rotor flexing w/o breaking. I guess any flex at the hub (front or rear) would be multiplied quite a bit with a 12" or 13" (or larger) rotor between it and the caliper.
Al, I think it's due to my mini tub and deep lip wheel in conjunction with my 335 tire. Matt at Morrisson chassis put together a drawing and said the load is about 2.25 greater than recommended factory loads on the bearing and axle flange. I've already got a 1" bore on but haven't driven it yet. Supposed to have a floaing caliper or rotor coming out later this year.
 
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