Front Left Wheel Locks Up Under Medium/heavy Braking

[UrQuattro]

Hey everyone. On my 81 targa, the front left wheel ALWAYS locks up first under "hard" braking. To me, it feels like it is happening very prematurely as all three other wheels are able to handle a fairly significant amount more braking force before lockup.

I don't feel that any of the calipers are sticking at all as it rolls very freely when at a stop.

The tire pressures are equivelent left to right on each axle.

My thought is that there is something going on with the calliper not being able to apply even pressure to the piston and therefore it is more prone to lockup, or it's the tire that is the issue.

Any thoughts on what I can check for obviously? The brake fluid was flushed before I got it back on the road in July of this year after inheriting it in '05 after my father died. But I think I remember noticing this issue before then as well.

Thanks in advance everyone

Michael

[Flieger]

Check the flexible brake lines to the calipers. They swell on the inside as they age and wear out. This can cause fluid to go in under the high system pressure but causes it to be trapped by the swollen lines and inadequate suction/return pressure.

FWIW, my car always locks the driver's side front tire first, soon followed by the passenger side front. I am quite sure my corner balance is off, I think by quite a bit. The drivers side front is high and the corresponding right rear diagonal is low, so there is not enough weight on that left front tire. The front tires are also more heat cycled/older/worn out than the rears.

You should verify both your alignment and corner balance.

[wdfifteen]

Originally Posted by Flieger

Check the flexible brake lines to the calipers. They swell on the inside as they age and wear out. This can cause fluid to go in under the high system pressure but causes it to be trapped by the swollen lines and inadequate suction/return pressure.

FWIW, my car always locks the driver's side front tire first, soon followed by the passenger side front. I am quite sure my corner balance is off, I think by quite a bit. The drivers side front is high and the corresponding right rear diagonal is low, so there is not enough weight on that left front tire. The front tires are also more heat cycled/older/worn out than the rears.

You should verify both your alignment and corner balance.

Check the right front for a binding piston or a pad hanging up or some reason. It's possible that the left front is working fine and the right front is not doing its job, forcing the LF to do all the braking and lock up.

[DanielDudley]

It can be as simple as a swollen line causing one rotor to be hotter than the other. It can be sticking calipers or glazed pads and rotors. It is for sure something, but not likely a corner balance issue. I have even seen mismatched tires cause this, on tire older than the other, one tire low pressure.

The brakes on your car should stand it on it's nose. I would be surprised if new lines, rebuilt calipers and turned or replaced rotors don't cure your problem. You need good brakes, and it is false economy to be going in there two or three times to check it out. Repack the bearings and replace the seals WYAIT.

BTW, don't let all the fluid drain out of the system while you are swapping out, but do flush the old fluid out. An empty system can be hard to bleed.

[911pcars]

You have some diagnosis suggestions thus far.

Before venturing too far and start buying new pieces, try solving this in easy-t0-digest steps. That is, do the simple stuff first to confirm it isn't something else.

Are front tires the same? Do they have the same tire pressure? Same wear pattern?

I'll not condemn the brakes at this point in time since you report the symptoms only occur under emergency-type braking maneuvers. If you have a sticking caliper piston or bad brake lines, symptoms will likely appear under normal braking as well as when coasting.

Perform a tripod test to determine the chassis is in balance. If it's not, a front tire can be lighter than the opposite side and lock up sooner. That's a distinct possibility and it doesn't cost you anything unless you don't have floor jack. See the archives for the procedure and theory behind this.

Hope this helps,
Sherwood

[SC-targa]

I agree with 911pcars last suggestion. My guess is the corner balance is off.

I had the same problem with mine when I bought it. I rebuilt the caliper and installed new brake lines to no avail. Then I found one front wheel was carrying all the load. The tripod method should get you fixed up.

Regards

[rootmatt]

Now that the one wheel has been locking for a while, it probably has a flat spot on that tyre. That will only make the problem worse.

[dad911]

How low is the car, if it is too low, hitting bump stop, you will lock front. Also has it been Corner balanced?

I had a track car, took forever to diagnose a front wheel locking. One front strut was binding.

[Steve@Rennsport]

Provided there are no issues with the braking system, I would have the corner-weights checked.

A level car can have up to 200+lb cross weight discrepancies that results in a "teeter-totter" going down the road. Needless to say, this is far from ideal.

[88-diamondblue]

i had the same problem and turned out to be a bad tire. Couldn't see it but once it was off and looked at the tread it had very uneven wear. Looked like a broken belt.

[UrQuattro]

Thanks for the input everyone. I forgot to mention that under medium/hard braking while slowing down from, say, 80-45 quickly, the car will pull to the left as well... The brakes were recently redone, though just with fluid, pads and rotors.

The tires have LOTS of tread left, but they are getting up there in age. I think they were put on the car in 02. The rear tires are new 225/50-16 kuhmo spt's. The fronts are Bridgestone Re710's i think... i'd check, but it's COLD out right now and wet... i'll double check later...

It has never been corner-weighted as long as i've owned it. Also, it is not TOO low as i cant remember the last time that I hit a bump stop under normal driving...

Anyway.. thanks again.

Michael

[Vereeken]

I had the same problem many moons ago. For me it turned out to be the wheelbearing left that was shot. I replaced both and now they look up at the same moment. Something to put on your list when all else has been ticked off.

Michel

[80-911SC]

if you haven't hit a "bump stop" you are not understanding what bump stop is ...
the 02 tires are way past usable ... new and old tires in a mix defiantly not a good idea ... have you checked your brake pad thicknesses from side to side for uneven wear and check the rotors for warpage and proper thickness

[UrQuattro]

Well, I do know what a bump stop is. It helps prevent excessive travel of the suspension which would cause damage and be potentially very dangerous.

What I'm trying to say is that I have enough suspension travel such that I am rarely (to the point that I am not noticing it) feeling or hearing the shock associated with running on the bump stops.

I don't remember what I setthe ride height at, but the friend of mine who did it for me knew what the "proper" ride height is.

As far as the brakes, as I mentioned above, the pads and rotors are new, with about 2.5k miles on em.

Am I missing something that you weretrying to get me to understand? Though I am not very knowledgeable about p-cars just yet, I do know cars pretty well.

I'm open to new info. If Im misunderstanding you please let me know.

Michael

[Flieger]

I think all the (many) possiblities have already been mentioned.

Change the rubber brake lines to the calipers first. They can look good on the outside but be bad because they swell on the inside when reacting with the brake fluid.

If not that, I would say the tires have heat-cycled/aged out by this time.

Corner-balance problems would probably be most noticeable with a very much lightened car, as the weight comes off the front easiest, shifting the weight distribution rearward. Even then, you should notice less lock-up from the left front when taking a right turn.

By the way, you have confirmed the lock-up on various roads, correct? The road crown/camber can have a big effect, especially on pulling to the left/right under braking. Tires also have a big impact on tramlining. Sticky DOT-R tires like Advan A048s do this quite a bit, for example.

[UrQuattro]

Yah, I've been able to replicate the problem on any road that I want to check it on. I haven't noticed any vibrations from the front, and I am really quick to react to the lockup and return to threshhold, so I'm not overly concerned with flatspotting. None of the lockups lasted for more than a fraction of a second. It's been a long time since I've raced, but some of my reflexes are still there.

Thank you everyone with the suggestions. It seems that a few things need to happen.

I should probably just go ahead and replace the brake lines all around with braided stainless, have the callipers on the front checked for any need to rebuild, get new front tires, OR - the other idea I had was moving my new rears to the front and then installing some 245/45's on the rears. Etc etc.

I have a lot of stuff planned for the car, but i'm just in a tight place for money in a big way.

Thanks again
Michael

[palle7688]

Have you checked your caliper pistons.
They should be turned so the milled of part, points in the same direction?
Acts like a servo on the pads.

[palle7688]

Tech article here on pelican has it:
Pelican Technical Article: 911 Caliper Rebuild
Item 14 and 15 ( A popular way to say it is: It should be "blowing into" piston if you see from the direction of the wheel rotation)
Check that!

[Flieger]

Ah, good suggestion. The 20 degree angle is important for even pad wear.