THSCC Forums

Ok, I have been battling a braking problem for sometime now. Here’s what happens:

Street: Firm pedal, great braking feel, can lock ‘em up at will with my Bobcats. Nothing to see here, move along….

Autox: Firm pedal at the line, but anytime I run it hard into the boost (ie running 2nd gear out), I go to hit the brakes and the pedal goes almost to the floor. You get braking power but it startles the hell out of you and its not what I would term “desirable” 

Track: Only noticed loosing the pedal at all braking after the front a back stretches. I didn’t wind it out to redline like I do autoxing. It wasnt “that” bad and it didn’t happen every time. Still not confidence inspiring…

What I have done to fix it:

Bled system 1398429857930589 times, Valvoline stock had to increase a few bucks from my brake fluid purchases
Stainless Steel braided brake lines
2 New Master Cylinders
2 different brake boosters
2 different vacuum hoses from intake manifold to brake booster (There is a valve in the vacuum hose that holds all the pressure my compressor will throw at it, its supposed to let the vacuum in and not let it out, it seems to be working fine).
Rebuilt all four calipers August of 03

I’m stuck here. I’m thinking of buying new Calipers all the way around. Rebuilding them seemed to be the only thing that helped any at all. I just hate the throw parts at a problem I don’t know will solve the problem. I know it certainly wont hurt to replace the 13 year old calipers on the car now.

Any thoughts? Solutions? I just knew that replacing the vacuum booster would do the trick but alas, it didnt. <-- That was the last thing I tried...

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02 Focus SVT
STF 9

How about Motul fluid and race pads (or maybe that's what bobcats are).

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Diane (Hall) Bundas
1992 Spec Miata #48 - 1997 Chevy Tahoe - 2007 Honda Civic Coupe

Ryan,

1) It sounds like a "travel" problem vs. a "pad or fluid" fade problem if I understood what you wrote correctly. Unless you are "power braking" as you accelerate you shouldn't have brake fade in a normal autocross.

2) If the pedal effort doesn't go up, then it probably isn't a "power brake booster" problem. "Boost" problems increase effort, not travel in my (limited) experience.

3) What will make the pedal travel increase is "pad knockback", especially with 4 wheel disc systems which "assume" tiny amounts of pad travel. Edit to add: This assumes that at least one of the new master cylinders was good.

4) Pad knockback is generally caused (again in my limited experience) by loose wheel bearings or very warped rotors or anything else that pushes one or more pads/pistons back into the caliper OR the caliper itself is being moved relative to the rotor. It could be that in heavy cornering situations such as autocrossing or open track that one or more pads is being pushed back. Since you've checked/replaced everything else. I would check for these items before spending any more money.

5) Another possibility, but not likely in this situation, is that one or more of the pad backing plates is too tight a fit in the caliper. This, in my experience, however, causes pad drag rather than excess travel.

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Dick Rasmussen

FS 50 2018 Mustang GT

Most pro roadracers do a brake check nearing the end of the straights to take care of knockback after riding the curbs in the previous corners. They just give it a quick stab to get the pedal back and continue on. Even in the big prototypes this happens not just the GT cars. If it is knockback and ALMS teams still don't have a solution looks like you are screwed.

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2006 Civic Si - #24 HS for 2015

2005 GMC Sierra
1991 318is Garage Ornament

An additional comment: The sealed late Model Mustang wheel bearings/hubs have a tendency to distort enough under VERY heavy cornering loads (Wide R tires in bumpy track situations with heavy versions of the car) that the rotor will be forced to bend . . . ultimately resulting in Brembo rotors coming apart. The solution for most Tracked Mustangs is to replace the hubs at reasonable intervals. Took Mustang folks awhile to figure that one out. Somehow I suspect that DSM's could also have weak hubs/bearings so unless yours are new you might try that. In the case of the Mustangs there apparently are "witness marks" on the rotor where it has rubbed on brackets it should never rub on. Distortion like this could contribute to pad knockback.

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Dick Rasmussen

FS 50 2018 Mustang GT

I dont left foot brake and its an "instant" thing. It doesnt get progessively worse.

Also, the passenger rear caliper is a bit difficlut to bleed. It requires much more pedal pressure to force the fluid from the bleeder screw. There arent any more bubbles coming from that caliper or for any caliper for that matter. I have little trouble bleeding my brakes.

Diane, Bobcats are Carbotech's new street pad. I had this same problem to a lesser extent with my Panther Plus pads. Im using Valvoline because Ron said that it wasnt "really" necessary to use the supa dupa brake fluids unless you like to bleed brakes often due to the hydroscopic nature of the high boiling point brake fluids.

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02 Focus SVT
STF 9

Both front wheel bearnings were replaced in October last year. I have never touched the rears. Hmmm I guess I really need to carefully check those puppies out. It is certainly reasonable to think that if the front ones died, the rear could be toast too.

Im not helping things much by running 9" wide wheels with a WHOLE bunch of offset (+24) on a car that came with 6" rims

Thanks for you help Dick, you are truly a valuable resource for us newbies.

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02 Focus SVT
STF 9

You're not the only one experiencing brake problems.

1) Do you have an FSM, if so, have you followed their procedures of what to test in what order? If you don't have the FSM, PM me and I'll send you what my book tells me for my car.
2) Were the parts you bought good, quality parts? I buy only factory parts as I've heard so many problems with non-factory parts and so far have been lucky.

I'll post my symptoms and let everyone know where I stand as I learn more on another thread.

phil

Ryan that offset REDUCES force to your bearings.

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2006 Civic Si - #24 HS for 2015

2005 GMC Sierra
1991 318is Garage Ornament

Ahhh, nope.

He is running less offset than stock, hence the wheels stick out further that did the stock ones. Which increases the lever action on the wheel bearing since the weight is further away from it.

It really sounds like a vacum/boost problem to me. Double, or triple or what number you are on , check your lines. The whole turbo thing you have going makes me think your problem lies there as well.

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91 Jetta GLI STS/DSP 111
85 Porsche 944 ASP 11
http://www.blackforestindustries.com

I guess I should say, during cornering the force is reduced. Yes riding down the highway it would be increased. However, if it was knockback less offset does reduce force to the bearings in a corner. Look at all Purpose built racing cars rear wheels. Almost all of them run low or 0 offset.

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2006 Civic Si - #24 HS for 2015

2005 GMC Sierra
1991 318is Garage Ornament

I still think you're getting boost in your brake vacuum system.

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In need of car.

Ryan:
I tend to agree with Dick R. If first stab at the brakes is deep, it is nearly certain that something has "moved" in the system, requiring an infusion of fluid to move it back into position.

After release of pressure in a typical disc braking system, the pad retraction from the rotors is measured in thousandths. However, if the pads are forced further out from the rotor, like by a badly warped rotor (not likely, in your case) or a wheel bearing that allows axial movement of the wheel/rotor assembly, they bear against the cylinder in the caliper and basically pump fluid back into the master cylinder.

Then you're back to the fluid requirement on the first stab, which get's your attention, of course.

Not that it would affect your stated problem, but I would investigate the "hard" pedal pressure required to squeeze fluid from one of the wheel cylinders you mentioned. Probably not normal. May be just a restricted/plugged bleeder screw. Try replacing the bleeder screw on that wheel cylinder. Good luck

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Brad

Ryan,

One more possibility that I really don't know whether it is possible with your car: Drive shaft in/out (axial?) movement "pushing" the wheel/rotor flange against the piston. Is your car AWD? If so, that gives 4 half shafts to check for movement. How do they attach to the diffs? Any chance there is end play in any of the axles and/or movement of some other part that would allow the rotors to move in/out against the pads?

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Dick Rasmussen

FS 50 2018 Mustang GT

Dick, your right, my Talon is AWD drive but in reality I have 7 shafts. The rear 2 and passenger front are 2 piece assemblies. The Drivers side front is 1 piece (those crazy Japaneese engineers). Lots of opportunity for play. I keep getting a haunting suspicion that its a wheel bearing for some reason. Theres a squeek that I just cant pinpoint....

Im gonna jack it up this weekend and check for axial play. Here is a dumb question, to rule out vacuum/boost problems, could I hook a vacuum pump to my brake booster? That would eliminate what Jason thinks my problem is. I can get a 12v vacuum pump from work (I think).

Thanks for all the help!

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02 Focus SVT
STF 9