THSCC Forums

From reading the descriptions of the problem it seems that the problem appears to happen after being on boost. I would look closely at the brake booster to see if there is not enough vaccumn in the booster at the end of the straight. The check valve is under a lot of differential pressure with boost on one side and vaccumn on the other side.

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George Bright Member # 141
One of the youngest of the old farts.
Yes I voted against you joining the club.

Bryan:

Did you find the solution? I favor the runout/knockback theory
considering you may have as much a 1/8 play in the bearings
you need to fix that first and go from there.

However, considering you & Matt said about
full turbo boost:
There should be a "check valve" in the hose running
from the intake to the power brake booseter.
Can the check valve be in backwards, broke, or missing?
Look this up in the shop manual and replace
this should a $2 part.

Other than that, I would be suprized if changing the bearing
did not solve the problem.

Comments on Runout/Knockback Management:
================================
I believe you should always measure run out
of your rotors to minimize vibration and or knock back. Excessive runout
will cause problems with stock brakes using sliding pins to mount
the calipers, but it will casue pure havoc on a aftermarket fix mounted
caliper on a fix mounted bracket with fixed hats. Floating Hats will act
a like the sliding pins on stock caliper mount but un-fortunatly, unless
you spend really big $$ on a Brenbo or Alcon kit you are going to get
a fixed mounted rotor and if you did the floating hats will not totally
prevent knock back either. DelcoMoriane used to put springs on the back
of Corvette calipers (that were fixed mounted) to prevent knock-back
But springs are not used on modern calipers, why I don't know.

For those with Big Brake Kits:
===================
The way to manage knock back, in addition to a slight tap on the brake
after a long streight, is to keep run-out to an absolute
minimum. Most after market kits have three surfaces to introduce
runout ( hub surface, hat surface, rotor to hat surface) as opposed
to only two (hub & rotor).

Most likely, if you just bolt everthing together you will
have large amount of runout even though the individual pieces are
all at acceptable tollerances.

What I do is take my rotor off the hat and start to re-index it untill I get the least amont of runout. (8 different positoins for a 8 bolt hat)
Then I take the hat/rotor assembly and re-index it on the hub untill
I get the least amount of rounout (5 positions on a 5 bolt hub). Then center punch mark & torque everything down and re-check. Naturally
this requres a dial indicator. Any decent mechanic doing brake
jobs will have a dial indicator so should you.

Check any spacers on a sheet of glass covered with 600grit sand paper. If a spacer is powercoated it will need to be block sanded, components
should be cad plated or adonized where the dementions are not changed
Power coating should never be used on a critical dimension part but I've
seen it done.

Adjust bearings to 0 lash or replace sealed bearings with any sign of play.
Purchase sealed bearings from either the dealer, Timken or FSK as they will have closer tollerances then the cheep "white box" parts.

I have measured run out as much as .025 and got it down to .00025-
.00050 using this somewhat tedious but effective method. If you
can not get it under .003 then one of the three component surfaces mentioned was not machined true and needs to be replaced.

With run out that low you will get very smooth stops under max
braking and no shake.

Interested to know what happened?

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Mark Vitacco
THSCC TT Chairman
mvitacco@bellsouth.net

I just wanted to give my recent experience with this problem on my blue RS & the solution.

Symptoms...

at VIR in February, I had to give the pedal a pump to make it firm for the braking zone after a long straight; not horrible, just annoying

more recently, it got worse
slight pad knockback when driving straight for a while
intermittent clicking when turning left
severe pad knockback after hard turns
sloppy steering

Here's what I did & what it fixed

replaced front calipers - fixed the sloppy steering (don't ask me why, it just did)

replaced right front wheel bearing, which had grenaded a couple thousand miles ago - fixed the clicking & pad knockback

replaced all rotors (worn down to min thickness) & installed new Carbotech Bobcats - it stops like Tom's POS with its BBK

If it isn't knockback...then I would put a test guage right off the master vac hose connection. See what is really happening.
You should have a vacume resivoir somewhere in the system...It will have check valves and there is another check valve near the master vac. One of these could be causing the problem. Also....you could be experiencing vapor lock in the brake lines....water dissolved in the glycol brake fluid can be driven off by the heat of race conditions....this can wreak havoc with your pedal feel.
Try switching to silicone fluid....although it is not as heat resistant as DOT3 glycol...It does not have the strong affinity for water that glycol does. You could drain and bleed the lines before every race...refilling with new dry fluid of course. I would bet that you cannot get all the residual water out of the lines on a daily driver without heating all of the components while you drain them. sounds liike a candidate for silicone to me.

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Paint don't make it go any faster.

Well he'd have to get a virgin system somehow to put the silicone in wouldn't he?? The Two fluids aren't compatable.

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2006 Civic Si - #24 HS for 2015

2005 GMC Sierra
1991 318is Garage Ornament

John,

I'm confused about your suggestion regarding using silicone fluid in a "performance" application. Everything I've ever heard or read about silicone fluid is that it is not good for performance applications. This link's comments are what I've heard consistently from performance oriented sources: http://www.rpmnet.com/techart/fluid.shtml#fluid

Is there a new version of silicone fluid (that I haven't heard of)?

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Dick Rasmussen

FS 50 2018 Mustang GT

well....there ya go....silicone is bad they say....more compressable when hot. The water problem is common to all poorly maintained systems....I just think that silicone fluid has no affinity for water and would be more servicable in a street performance car. My thought was only that with less common fluid changes...you might not have the drastic vapor lock problems associated with water laden glycol.
Virgin would be ideal. You can flush the glycol with a whole lot of silicone and use the current brake system. You must heat the brake components when changing any types of brake fluid to make sure you expell all the water. A flame spreader and a pocket pyrometer will help you to heat components without burning rubber seals. The lines are obviously not going to be reachable....just heat the calipers and MCyl. some even heat the new fluid to keep it dry while handling. Flush entire system with LARGE amounts of new fluid.
With glycol....you may have to do this every race...
With silicone...it might be only once a year.

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Paint don't make it go any faster.