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A little more background info for those trying to help us solve this problem. The problem first surfaced at the last two day event in Rocky Mount last season. If you remember the course, there was a tight slalom at the start then a couple of offsets before the C-box. Going into the C-box the pedal goes to mush. Everywhere else on the course ithe brakes were fine. WTF?

So afterwards we bled the brakes again. Not much air to speak of except the right rear caliper. That one always seemed to have more bubbles for some reason. We thought we were good to go and were ready for the NCAC 24 Hr event in SC.


At the 24 hr NCAC the pedal felt fine on the drive down and in the pits...hard as a rock very little travel or effort. We tried different combinations of staying in 1st gear for the first part of the course or shifting to 2nd. Going to 2nd gear earlier "seemed" to make the brakes better, but not great. Near the finish there was a hard right (around a pole) then a little offset near the finish beam. On my first pass the pedal went almost to the floor in the hard right, but seemed to bite right at the end of the pedal stroke, almost too late. After the right I was in the boost in 2nd gear through the finish lights where again the pedal goes to the floor and finally bites enough to lock-up the tires. This was a good thing since the moron who designed the course put the finish right in-line with the cars lining up for start. Had the brakes not locked up I would have been stopped by a nice 6" high curb just ahead.

We scratched our heads the whole weekend trying to figure out what to do. I combated the problems on my runs by giving the pedal a stab before the hard right hander and again before braking for the finish. It wasn't great, but we made it through the event at least.

Afterwards Ryan rebuilt all four calipers during which he found what seemd to be the problem with one of the rears. The rest of the year the brakes seemed pretty good, but still not what we would call great.

The problem re-surfaced again in Laurinburg this year so we decided to go ahead and buy a brand new factory master cylinder. We did the bench bleeding with no problem and all seemed well in Danville, though there was no hard braking zone on that course.

At Sanford all seemed well until entering the first turn around and again the pedal went to mush. The brakes seemed OK, but not great on the approach to the lane change immediately afterwards.

If I'm understanding the braking system correctly, the brakes (pedal travel wise) should be the same even with no vacuum booster at all...correct? I don't think the booster has anything to do with it, nor the brand new master cylinder. But we can't figure out what it is so what do we know! HELP!!!!!!!!

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Vincent Keene
'06 Ford Mustang GT (track rat)
'15 Dodge Charger R/T (yeah, it's got a HEMI!)
'07 Ford Fusion SE (205,000 miles and counting)
'98 Chevy Z-24 (retired)
'93 Acura Integra (Team SWB 24HOL Car)

You might ask Ron what he used for pads and fluid on his Talon. My first suggestion for the track (since this seems to be where the bulk of the problem is) is try a non-street pad. That's a heavy car you've got there, and it isn't particularly slow. Heavy fast cars don't last long on street pads on the track.

The brake fluid thing... well I think you'll likely get as many opinions as there are people. In the Miatas (I know... apples and oranges), I didn't like the mushy pedal I always had after track events with the Valvoline-range stuff. Ick. So I tried the Motul and never went back. Of course with the Miatas on Motul, I bleed the brakes about once a year because I think I ought to. With the other stuff, I had to bleed them more often because I didn't like the pedal feel. I think in the long run I come out better using the Motul. YMMV.

Seriously, give Mr. Spencer a call. (I don't know how often he checks the board.) He tracked one of those things for several years and might be able to head you in the right direction.

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

Well, I think the reason we get more air from that caliper is its the first one we bleed.

Could a bad proportioning valve be the problem? One of the guys on the DSMAutox group mentioned his went bad and caused some screwy problems. I have a spare valve on my parts car, so changing it is a freebie. Is it worth the time?

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02 Focus SVT
STF 9

[quote="Ryan Holton
Could a bad proportioning valve be the problem? One of the guys on the DSMAutox group mentioned his went bad and caused some screwy problems. I have a spare valve on my parts car, so changing it is a freebie. Is it worth the time?[/quote]

Ryan,

Maybe. I didn't know that any OEM four wheel disc cars used proportioning valves (thought that was for disc/drum systems and race cars). The few I'm familiar with do it with relative pad and piston sizing. However since you have one, I guess it is possible that with a bad valve that the problem only shows up with higher pedal/line pressures "forcing" the rear system to work . . . and if the rear system has air in it because it won't bleed properly . . . As far as worth a swap, it depends on the odds of the parts car valve being healthy.

What I do when diagnosing brake problems is do some gentle street driving/braking and then feeling the relative heat of the wheels (rotors are too hot!!). If the rears are cold after some braking try being a little more aggressive to see if that gets some heat in the rears. Also, how does the parking brake work on your car? If it involves the rear brakes there may be some effect on normal braking if something is wrong.

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Dick Rasmussen

FS 50 2018 Mustang GT

[quote="DickRasmussen

Ryan,

Maybe. I didn't know that any OEM four wheel disc cars used proportioning valves (thought that was for disc/drum systems and race cars.[/quote]

Really?

Almost every car I have ever worked on has had a proportioning valve, and most have had 4 wheel discs.

You need to get away from the ford junk and work on a real car

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

Ryan,

Here is a very complete (and hopefully correct) description of proportioning valve operation. Based on this description, pressure is reduced to the rear brakes under heavy (i.e. panic) braking.

http://www.babcox.com/editorial/bf/bf50014.htm

Adam,

Edit: Because I was stupid (or at least ignorant)!!!

My Mustangs actually do have proportioning valves. They also have ABS. What they don't have is a mechanic who knows his stuff.

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Dick Rasmussen

FS 50 2018 Mustang GT

My ABS computer does the front-to-rear brake proportioning for me.

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Has no responsibility whatsoever.

Aren't sliding piston calipers designed so you DON'T get pad knockpack? Also you seem to be getting this after straights, and not turns. Knockback is caused by lateral Gs and components flexing.
http://www.stoptech.com/whitepapers/knockback.htm

Did you replace the caliper pins during the rebuild? I had mine get "worn out" somehow. Even though I regreased them, they somehow "busted" and the caliper didn't have any more sliding action. I just picked up some used 10K mile calipers off a wrecked WRX. Don't they use the same calipers on the non-GS eclipses or did they start using the two piston ones on all of them. You might be able to pick up a low mileage set from a junkyard. If that fails, picking up a set of remanned calipers from thoff or someplace like that wouldn't be a bad idea. I called T-Hoff and they are $120 for the front and like $240 for the rear our price. It would kind of suck to pay $360 for new calipers, but I don't believe this to be an issue with something causing pad knockback.

-Tom

Tom,

It is true that knockback is probably happening in the turns. However, "usually" you don't apply the brakes as you exit a turn leading onto a long straight . . . I think you are correct about floating calipers being less sensitive to pad knockback. I don't know if they eliminate it.

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Dick Rasmussen

FS 50 2018 Mustang GT

Update

Ok, I checked the play in the wheel bearings and the drivers side rear has a SLIGHT bit of play (less than an 1/8"). Is that enough for the pad knock back theory?

Also, I found the brake proportioning valve and it looks as if has leaked sometime in the past, it was all cruddy on top but it was dry now. The propotioning valve out of my parts car is cruddy on the bottom. Think this is worth replacing? Im gonna see what one cost tomorrow.

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02 Focus SVT
STF 9

Ryan,

What is the spec? Approaching 1/8" sounds like a lot. Between the link that Tom H. posted and the one that I posted you will probably know at least as much as the rest of us regarding to what to try next . . . especially since you can apply all that "book learning" to your specific car. Thanks to your question, I've certainly learned a lot.

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Dick Rasmussen

FS 50 2018 Mustang GT

Im gonna let someone else wiggle the bearing before I replace it. I have a good idea its the problem. The service manual limit for play is .8mm or .031 inches, so pretty much, if it has play, it needs to be replaced. Most others find the rear ones are easier to do than the fronts, I have replace 3 fronts already, so hopefully it will be a breeze.

My proportioning valve looks a lot like this:


Im gonna replace it for good measure. I do plan on tracking this car, so I dont see this as money wasted, just an ounce of prevention.

Thanks everyone for your help! Im on vacation next week, so Im gonna HOPEFULLY get this all solved, I'll post back and let everyone know what happened.

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02 Focus SVT
STF 9

Ryan, have you solved the problem yet? Was it the bearings?

Would worn bearings cause excessive wear on the Inside pad? My last set of pads wore completely out on the inside but the outside pads were almost brand new.

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'99 Mk IV Jetta VR6 now claimed by the better half.

'90 Miata A-package, Floats like a butterfly and buzzes like a BEE with loudspeakers.

Doug,

I think the pad wear is the clue to solving your problem. Inside vs. outside pad wear should be very close to equal unless something is wrong with the caliper.

Is pad wear more rapid than "normal" recently?

Assuming floating calipers (pistons just on the inside of the rotors) wear on the inside to me implies that either pistons are "almost seized" and not wanting to release pressure or the caliper sliding/moving parts are stuck. In fact, I would think the caliper being stuck is the more likely cause of both your problems.

If a caliper slider or piston is stuck the brakes/wheels probably will stay VERY hot even if you do a test drive with minimal brake use. You might feel excessive drag if you press the brake pedal with the wheel in the air, release the pedal, and turn the wheel. If it is a rear wheel the parking brake could be sticking in some way (assuming rear wheel parking brakes).

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Dick Rasmussen

FS 50 2018 Mustang GT

Ryan

I remember the *exact* same thing when I drove Ron's old Talon years ago. The brakes were stellar in the pits/grid, but after being on boost, the pedal would go almost to the floor before anything happened. Scared the sh!t out of me the first time it happened. Got to be something about the vacuum system.

Matt