Help Needed

Dave, lots of things to consider. Is there a difference when hot or cold? From your post I am not sure if you did the rear brakes as this could cause a pull. Now pulling to the right could mean that the right side is grabbing slightly more or the left side is not providing the proper amount of force. A quick test is to use a thermo gun and after some good brake applications take each wheel temp and compare to the others. Depending on the results it could show a wheel not working as hard as the others and if temps are the same you could be getting suspension movement maybe at the rear causing the car to change direction. Other causes related to high temps are the pads binding in the guides and not fulling releasing, but a temp gun will narrow this down as it will show a higher temperature to the rest of the wheels. Anyways just some thoughts to help narrow the problem and isolate the location.

I would take a guess and say that the piston in your right front brake is not returning all the way back when you step off. This is leaving enough pressure on the pad to give drag and make the car wonder to the right.

If as you say, 'everything was done perfect' try switching the left and right-side front rotors and see if the 'pull' changes sides. G.M. sources their parts from a host of suppliers, and cast iron from different batches sometimes has different coefficients of friction. On some 'Vettes, this has resulted in pulling in one direction with all-new parts installed by dealers, when one new rotor came from a Michigan foundary and the other from Alabama. This is also why you can run into trouble changing only one front rotor at a time. Brake pads don't exhibit this difference.

Derrick,
Yes, I checked the rears to make sure they were not causing any of this. Also checked all the wheels. Did the temp check after testing and there was a slightly higher temp on right side. This led me to think the pistons were sticking on the right side. But everything we found after indicates that was not the case. The problem is the same hot or cold, but when hot and as brakes grab this becomes more severe. Could not find any movement anywhere in the suspension or steering - even with a serious prybar. All is tight and all bushings are perfect.

Doug,
Pistons perfect and identical gap throughout the application and release of pressure.

Boss,
Both discs were resurfaced now and measured. Also identical. But you may be right about materials / friction. I changed the pads from right to left, thinking the pad material may have been different - but this made no difference. Could it be the disc material itself?

Any more ideas?

There is a gauge that I have that measures brake fluid pressure. I very rarely have used it.

At this point it would be worth considering getting one. If for some reason you are getting unequal fluid pressure that would be the cause.

I don't know if the original proportioning valve was removed from your car. I presume that it was.

I would look at your system for something that could cause uneven braking pressure including a small leak in the lines that you are not aware of.

Incidentally do you have one-man bleeder valves on the calipers? If you open them too much they will let air in. Air in the line would also create uneven brake pressure.

We took very accurate brake pressure measurements on all four wheels. After the calipers AND before. I also took pressure readings thru the stroke. The difference was less than 50 lbs (the gauge only measures 100 lb increments).

No brake line leaks. No open bleeders. No air in system. Master, reservoir, booster have all been gone through.

Seemingly impossible. Let's see if I can get this close? "When you have eliminated all the obvious, what ever else remains, no matter how improbable, must in fact be the answer".

Two other avenues left. 1)sell the car 2)cut a hole in the floor and drag your feet.

I don't know what is left?

Maybe you are just blinded by all the chrome?

I'm sure someone will come up with something I overlooked. I will try swapping the rotors. I will also go for an alignment, although I can't believe that any alignment issue would show up only under braking.

I think your master cylinder is polished too much.

Are you sure it's not a suspension problem causing it?

Ron

How could I tell? Nothing moves although I only used a 3' prybar. Steering, ball joints, rod ends, bearings. No movement at all. All looks right. Bushings seem perfect, no swelling anywhere.

It could be alignment, but hard to understand how since it tracks perfectly straight.

Is that Bias adjuster plumbed on the fronts or rears?

Julian,
I had this happen on my Ferrari, turned out the front left in my case, Koni was failing. The car would dive more on the left, hence pull left. Not like you can just push an a fender to figure this out. My tech pall suggested it. Pulled the shocks and swaped...yep. problem on the left solved, now on the right. Sent the shock pair to Koni had them rebuilt.
hope this may help.
Daniel

It isn't an alignment problem. A bad lower ball joint or a bad outer tie rod can create something like that. More of a bumbsteer feeling though.

I don't think it is the strut either. The strut would be failing in compression to cause that. It would show just driving down the road. When the right would go over a dip the car would vere to the right. It isn't doing that.

Put new rotors on it or do what Boss wrench suggested. Swap the existing left for right and see if it pulls the other way.

What does the wear pattern look like on the pads? Are they glazed? Are there hot spots (high spots) showing on the rotors?

I see two pressure regulators in the picture. The fancy smancy one with the nice knob (I like nice knobs ) and the stock one on the floor? And you said that the pressure measured equal? Then the problem has to be in the clamping power being unequal?

Thanks guys. Good stuff!

Shock failure just might be it! I will certainly try swapping them. I did make sure they were dialed in on the same settings (top and bottom) but didn't think about whether either was failing.

At speed it is very stable over bumps. I can take my hands off the wheel flying over railway tracks or bridge expansion joints. But when crawling to a stop I have noticed a severe pull to the right if my right wheel sinks into a deep recess or pothole. Could a problem shock perform okay at high speed but reveal potential failure at slow speeds?

FYI - I have no control over rear brake bias. The zoomy knob just lets me dial in the fronts.

quote:

At speed it is very stable over bumps. I can take my hands off the wheel flying over railway tracks or bridge expansion joints. But when crawling to a stop I have noticed a severe pull to the right if my right wheel sinks into a deep recess or pothole. Could a problem shock perform okay at high speed but reveal potential failure at slow speeds?

FYI - I have no control over rear brake bias. The zoomy knob just lets me dial in the fronts.

From your statement it certainly sounds more like a suspension/shock problem somewhere. When checking alignment do you check for equivalent camber and caster not just toe?

Off topic a little but your front-rear bias is adjusted by restricting the front brakes? Most I have seen are in the rear brake circuit, so you have full front power and then adjust the rear to suit.

It will be a while until the snow melts, so I have lots of time to ponder the problem and dream about living in the warm parts of the planet. Like the deserts of Nevada.

Hey wait... didn't I just see snow in Vegas?

Joules, I need to look at the brake plumbing again. You could be right.

I think Daniel nailed it, it could be a bad shock. It doesn't even have to be a front shock, it could be a rear shock too. On the race car (not a Pantera), a bad shock will make the car twitch and turn and behave horribly under braking while it still seems ok cornering. I thought I had a major problem at the front of the car and it turned out to be my left rear shock.
And Julian is right too, you want that bias adjuster on the rear brakes. Most of your stopping power comes from the front brakes, so you want full pressure up front. You then use the bias adjuster to limit the rear brakes from locking up. Weight transfers forward under braking and the rear of the car can get "light" and lockup. It's also why a bad rear shock could cause your problem too. Weight transfers diagonally, so a bad left rear shock will load your right front tire more under heavy braking. So I would try swapping the shocks side to side and see what happens. If you want to get fancy, a race shop should have a "shock dyno" and could test your shocks for you.
And make sure your alignment is good and a rear tire isn't toed out. If a rear tire is toed out, your car will "crab" or "dog-track". You probably couldn't feel it driving on the highway, but under braking you would have to counter-steer against it and there's your "pulling" problem.
A good alignment shop can also check what I think they call "thrust angle". Which means the rear wheels are in line with the front wheels and not skewed.

could the suspension / bushes be worn ? when you pull the brakes the whole assembly moves ?
it's a long shot.....
good luck

Quote:

"I see two pressure regulators in the picture. The fancy smancy one with the nice knob (I like nice knobs Big Grin ) and the stock one on the floor? And you said that the pressure measured equal? Then the problem has to be in the clamping power being unequal?"

The "stock one on the floor" is the pressure differential valve. It only detects large pressure differences between the front & rear brake systems & turns on the waring light.

Yes. It's a distribution block. Same as in the Mustang.

The point was that there are devices in the line, like the adjustable proportioning valve.

Both should be looked at for correct routing. (:

I worked in a brake and front end shop for years. Every time this happened it was the suspension movement. Check bushings and attachment points. Braking forces will cause castor/camber changes that are hard to detect by jacking and prybar checks. If you pull the shocks the unloaded suspension is easier to check. Start with tightening all the bolts and swap tires right to left.
Clayton

quote:

The "stock one on the floor" is the pressure differential valve. It only detects large pressure differences between the front & rear brake systems & turns on the waring light.

It is actually a shuttle valve, if one brake circuit fails the valve will shuttle and shut off supply so you don't lose all your brake fluid and thus it keeps the other circuit braking. It will aslo turn on an idiot light on the dash, but you probably already know somethings up by then

quote:

It is actually a shuttle valve, if one brake circuit fails the valve will shuttle and shut off supply so you don't lose all your brake fluid and thus it keeps the other circuit braking.

Actually, the "valve" only "shuttles". It does not shut off the supply. There is even a small slot in each end of the "valve" to allow fluid to pass. The master cylinder reservoir has a small height "dam" between the front & rear fluid reservoir ports. If all of the brake fluid were lost in one brake circuit, the reservoir "dam" would retain enough fluid for the other brake circuit to function normally.