Interesting Quandry

[AFJuvat]

I have a bit of a stumper that I'm trying to solve. Was wondering if anyone else has seen a problem like this before.

1986 930

Problem: Drivers side front brake locks up.

Corrective action taken thus far:

Calipers Rebuilt
Wheel bearings replaced
Pads replaced
Master cylinder replaced
Brake hoses replaced less than a year ago
system bled several times
"known good" caliper installed
Rotors less than 1 year old, no visible defects, within specification as far as thickness and runout are concerned
Brake lines are not obstructed

It has been suggested that the corner balance may be off. This is a possibility (cannot determine until I can get the car on the scales). However, the car does not show any other symptoms of incorrect balance (odd ride height, binding in the suspension, etc)

Tomorrow, I am installing a "known good" hub and rotor.

any ideas\suggestions greatly appreciated.

AFJuvat

[autoxracer31]

i have the same problem, except mines on the passenger side.... What gives???

[ubiquity0]

I think your corner weights could still be off without showing up in unequal ride heights.

[AFJuvat]

bump

[BURN-BROS]

Does it lock up when you brake in a straight line or during cornering?

[Thomas Owen]

Question - does the brake on the drivers side drag after pedal application? In other words, can you detect any noticable difference in the temperature of the rotor from one side to the other? And, as BURN asked above, does this occur in straight line, left corner, right corner?
Regards,

[Wayne 962]

Need more info about the lockup...

-Wayne

[trj911]

I had a sililar problem and it ended up being a malfunction in the master cylinder. It didn't show the normal signs of leaking, instead it would only partially work, part of the time. Quite frustrating. I replaced it and the problem is gone. But, since you replaced yours already, I doubt if you have the same problem with a second unit.

[BGCarrera32]

Somebody needs to explain to me the whole "corner balance causes brake lockup theory". The caliper rides on the strut assembly, the rotor turns through it...so what diff does it make what the strut movement/postion is doing...now when the car is in motion I get it, but not just lockup and not releasing...

[ubiquity0]

If the weights are out of whack (2 wheels on the diagonal carrying significantly more weight than the other diagonal pair) the lighter wheels are going to have less traction, therefore will tend to lockup sooner.

[Zeke]

Originally posted by BGCarrera32
Somebody needs to explain to me the whole "corner balance causes brake lockup theory". The caliper rides on the strut assembly, the rotor turns through it...so what diff does it make what the strut movement/postion is doing...now when the car is in motion I get it, but not just lockup and not releasing...

Less tire adhesion on lighter wheel=locking up with same caliper pressure as heavier side. Look at race cars going into corners, always the inside (lighter) wheel that locks up.

[AFJuvat]

More info:

The brake locks up at any speed under any driving condition. The brake will lock at the end of pedal travel in normal braking, much quicker with "spirited" braking"

No significant remperature difference between rotors (caliper and wheel bearings were just replaced last week)

This has been an ongoing problem with this car - the car did sit for approximately 5 years. The car now has 50K miles on it.

The only items in the brake system that have not been replaced within the last 12 months are:

Rotors
Brake booster

AFJuvat

[GrindingGears]

tire pressure?

[OldTee]

Check the same side rear tire, parking brake adj. Go out and really slam on the breaks a couple of times.

[fintstone]

Sounds like a pinched brakeline.

[Wayne 962]

Define "lock up" please?

-Wayne

[BURN-BROS]

Was the master cylinder bench bled before install?
How well did you bleed?
Have you inspected all hardlines for damage?corrosion?

The rears may not be working as efficiently as the front. Your front left caliper lock up may be because its the only one working properly. You may still require a more intense bleed. The whole system has literally been gone through so that all I have. Hope it helps

[IROC]

Here's another vote for out of whack corner balance. I adjusted my ride height years ago by simply "eye-balling" it and noticed afterwards that my left front would lock up under moderate to heavy braking. I suspected a brake issue, but happened to corner balance the car shortly afterwards (it was waaay off) and the problem went away.

Mike

[na2ub]

Could it be that your left front is locking up because your right front is not functioning correctly? It would stand to reason that if your right front caliper was shot, or the hard line to the right front was obstructed or pinched, that when you push on the peddle, more force goes to the left front. Just a thought.