Seeking ideas concerning what may be causing the car to pull to the left when braking.

This is in the category of the IT help desk asking if you have rebooted your computer, but simple things first... is the air pressure equal in the front tires?  Any chance one of your new tires has a weaker sidewall than the other?

Beyond that, no solutions, but more troubleshooting ideas... take a friend for a ride leaving your jack, jackstands, and gloves ready (and maybe a second friend).

Get things up to temp.  Park it.  Jack the front end up to get both wheels off the ground, then turn each front wheel while your friend slowly depresses the brake.  If the left grabs first, you know it's grabbing early (or the right is grabbing late).

If they grab at the same time, is there any way you could you have a more friction between pad and rotor on one side over the other? Are the pads the same type and age?  Same question for the rotors.  Did you spill any fluid on the right rotor?  Perhaps try swaping the pads from the left side to right to rule out the pads (if that is possible on the stock brakes)?

When the brakes are dragging., Crack open the bleeder on the hot side. If when the fluid releases if the wheel spins free most likely your brake hose has collapse and cracked the lining of the hose and acting as a valve holding the pressure on that caliper. Replace both hoses.  I see it a lot on customer cars when they do a brake job and hang the calipers from the hose. 

Do you have the rubber flex brake hoses still? these will deteriorate from the inside and can restrict flow. Maybe one side is bad?

It certainly sounds like something is dragging or not releasing given the heat in the wheel. Shoot a laser thermometer at the wheel and rotor, see which is hottest this could be a wheel bearing failing issue that is dragging one wheel and exacerbated under braking.

All the above are worth checking, but a huge proportion of brake-pull in a Pantera comes from one piston in one front caliper sticking from varnish build-up. Pull a front wheel and have someone step on the front brakes while you watch closely for ALL pistons on one side to move at once. Then check the other side. There are 8 pistons and only ONE slightly stuck will cause a pull. Its caused by a car sitting dormant for long periods of time combined with the brake fluid NOT being changed at least yearly to eliminate moisture build-up. Fluid leaks will also do this but its far less common than plain ol' varnish.

The only possible fix is simple and cheap but is also one of the sloppiest, most miserable ways I can think of to spend a weekend: R & R each caliper, pump out all 4 pistons, scotch-brite the chrome plated piston surfaces & inside the bores, and reassemble, then bleed with fresh fluid. Do NOT separate the caliper halves and do one side at a time so you always have an assembled model. Costs nothing but labor (and several showers) but there will be quite a bit of effort! Some vendors will do a core- exchange on your stuck calipers; call around.

Just fixed a similar problem, expecting to rebuild caliper but found front wheel bearing seal leaked on to disk causing imbalance in friction. so new seal regrease and washed all parts with brake cleaner, all good.

Thank you very much for all your replies and interesting information. 

@bosswrench : I ALREADY replaced the seals and the pistons, and addtionnally lightly sanded the bores, fitted everything in place after smearing some brake grease... and before putting the calipers back, I checked that all pistons moved freely. That's why I was quite surprised AND disappointed !

I checked the tires, yes. There was also no evidence of any grease nor fluid leak, nor pinched or damaged flexible hose, otherwise I would have solved this kind of problem in first place : so, I'm in a fix :-€

You are right, a flexible hose could be damaged without external sign, so maybe I should replace the left and right ones. Except them, I can't see what I could replace in the braking system ! A step forward would be to replace the calipers (maybe one is distorted by the building heat ?), but it has a cost...

As a - new - start, I think I'm going to have the car partially tested on a DOT/MOT station's rolling bench, in order to measure the difference between left and right drag force. Even if I'm 99% sure that this is a brake-related issue, it coud help to exclude any front suspension issue (kinematics, wheel plane movement under effort and so on).

Matt

You've covered most of the common possibles, my friend. Other things that sometimes causes sudden steering problems is tire inflation pressure differences, alignment shifts especially on high-crowned secondary roads and internal brake hose partial-failures. Unless you are an originality;aficianado, I suggest a pair of dash-3 braided stainless or braided Kevlar hoses to upgrade the rubber stockers. 16" long fits all 4 positions. If you use ss-braided, they will machine metal off cast wheels so watch the routing. The vendors sell the aircraft grade hoses with a fitting that screws right in to the calipers and the stock junction block, so no jumping thru hoops to get things re-attached and no great expense.

On the weird side, one guy found that in replacing his rotors, he had received parts from two different casting runs. The coefficient of friction of the cast iron was different for the two rotors. He only found it by swapping the brand new rotors side to side.... and the 'pull' followed one rotor position. He was sensitive enough to feel the change in friction coefficient. I suppose the same thing might be possible with aftermarket brake pads, too. And there's always the possibility that the right side rack bushing may have come loose from its pressed-and-screwed-in-place position inside. That will allow the right side end of the rack shaft to rise and fall while turning the wheels, giving an 'odd' feeling. Good luck- J DeRyke

Hello guys.

After many months waiting for parts, bench availability and so on, I wanted to share with you the result of the investigations : at last, the culprit was the brake limiter.

My mech tried to find another part, without succeeding (it took him months... for nothing). He replaced it by two single limiters (one on each front line), setting them up on a rolling bench.

By far not the smartest way to solve the issue from an engineering point of view, but at last it works, and now my Pantera brakes w/o pulling.

Nonetheless I kept the OEM limiter, because I read somewhere that it could be recommissionned. Later, maybe...

Thanks again for your information and help.

Matt

Matt, are you speaking of the stock non-adjustable brake proportioning valve up by the radiator, or the equally useless stock dashboard brake warning light electro/hydraulic switch mounted under the power brake booster?

There's also a 'keep-alive' valve (AKA a rubber 'duck-bill' valve or sometimes a tiny ball bearing & spring) inside stock brake master cylinders, underneath the  front brake line fitting's conical seat. This normally maintains 1-2 psi in both front brake lines to keep the brake pads very close to the rotor surfaces for faster pedal response.  You have to pull the conical fitting seat out to access it.

All three valves affect both front brakes simultaneously thru hard & flex lines, so I can't think of a way only one  front brake would be affected by any of them; is your brake plumbing stock? A photo of the errant 'limiter' would help me understand, maybe. THanks-

Thank you for this information.

Yes, that's correct : I'm talking about this stock non-adjustable brake proportioning valve up by the radiator (on the left side, near the brake master cylinder). And as far as I know, the plumbing was stock before the mech modified it.

I, too, can't explain what could have caused the issue, as it doesn't seem to have a different action between left and right side.

The fact is, the issue has disappeared, and I wasn't able to reproduce it : maybe there was something wrong in the stock lines that have been deleted ? I really don't know... 

Matt

 

 

 

Matt, do you mean the proportioning valve was at fault? 
What do you mean by 2 single limiters? Can you give more details?
Thanks

Yes, this was the conclusion of the mech (he told me that this kind of issue had already been experienced, including other models than Pantera), and fact is that after its deletion, there's no more issue... but from a strict logical point of view, it doesn't prove that this part was at fault ! I guess that something else was modified (don't know what) and solved the problem.

He simply put one single limiter on the left hard brake line, and another one on the right line, and balanced them on a rolling bench.

By single limiter, I mean a small device allowing to adjust/reduce the output pressure level by turning a button.

Matt

I had the very same problem. I tried everything under the sun and then some more. Sometimes it pulled to the left, and sometimes to the right. I never thought of looking at the proportioning valve (which is still installed) because the proportioning valve reduces the pressure to both front wheels. In other words it splits the brake line pressure between front and back and not left and right. My problem fortunately resolved itself, but I never really found the cause. Maybe some of the more knowledgeable members can chime in and tell us if a defective valve could cause an uneven brake action on the front wheels and for that matter the rear wheels too.