Brake lock up

[rgoodrich]

I have an 81 SC. I had problems with the front left tire locking up way before stopping.
My mechanic found different brands of pads front to rear. He replaced the pads with Porterfield street pads. Now the left rear locks up, and the rear tires dust much more than the front. 8,000 miles ago, the entire brake system was replaced: New calipers, turned rotors, new MC, new vacuum booster, new SS lines.
I know that I shouldn't be hitting the stop pedal hard enough for any lockup, but on hard braking there is only a slim on-off margin before lock up.
Many years ago I had an 83 Euro SC cab. (Why did I ever let that car go?) I don't remember similar issues.
Any idea what to try next?

[equality72521]

I'll bet your corner balance is out of whack.

[CCM911]

I had a locking problem with the rear wheels when I first bought my car, and was wearing out rear pads more quickly than the front. It turned out to be the brake proportioning valve. If I remember correctly, it was located inline and was accesable via the smuggler's box. Once replaced, my brakes worked perfectly.

[SpeedracerIndy]

Originally Posted by CCM911

I had a locking problem with the rear wheels when I first bought my car, and was wearing out rear pads more quickly than the front. It turned out to be the brake proportioning valve. If I remember correctly, it was located inline and was accesable via the smuggler's box. Once replaced, my brakes worked perfectly.

SC's don't have proportioning valves from the factory, but that's a good thought.

The picture of the car looks like it's lowered quite a bit, so I would guess corner balancing is off as well.

[bfunke]

Check for fluid leaks and consider replacing your flex hoses. If they deteriorate on the inside they can cause brake locking.

[Flieger]

Corner balance solved my front outside wheel lockup issues. The rear was way low and 5 degrees different side to side.

[rgoodrich]

The car has not been corner balanced, so that sounds like a good idea. Will move that up to the top of the "list." I have some Turbo tie rods in a box, so maybe now's the time to install them, then alignment and corner balance. I'll post my results.

[Mitch Leland]

A long shot, but if your rubber brake hoses are old they can sometimes have little flaps that breakdown in the interior I.D. of the hose. So when the fluid is pushed through the hose the brake fluid doesn't return because the flap (hanging chad) acts as a one way valve.

[rgoodrich]

Sounds plausible, but the lines have been upgraded to Stainless steel from our host.

[equality72521]

Search for corner balancing and you'll understand why it's the most likely problem.

[Mitch Leland]

Originally Posted by rgoodrich

Sounds plausible, but the lines have been upgraded to Stainless steel from our host.

Well that's one item you can check off... It would seem to me that you would have to have a lot of weight over one wheel before the "corner balance" would come into play. But I have no experience to back that up...

[rgoodrich]

Car is now aligned, corner balanced raised to 24.5/25, and has new turbo tie rods. Also, new Bilstein shock inserts up front (Sport) and back.No more lockup, and completely different steering response. It's like a motorcycle now. Amazing how well a Porsche works when set up properly.
Thanks for all the input, guys!

[Wil Ferch]

And when you corner balance ( thanks for the tip, racer Randy Blaylock).....you may opt to go non-"ideal"....and purposely weight-jack the car to end up with nearly equal LF and RF weights. Ideal corner balance won't target this. With equal front weights, you will have less tendency to lock up one vs the other....at the compromise expense of turn-in characteristics to be slightly different with right vs left "turn-in".

Altogether....a good compromise to shoot for, for a race car...and I think even more so for a street car. You encounter threshold braking scenarios a lot more than you encounter threshold cornering scenarios.