Help with brake diagnosis, please

[sbmackie]

Car is an 82 SC, used for Auto Cross (driven hard), occasional track days (very few and I don't drive it that hard) and pleasure.

Replaced brake pads and adjusted/packed wheel bearings last week done by local pros. Super blue is 3 months old. On track yesterday, halfway through second session, I get a chatter in the wheel which get's markedly worse with brake application. One more lap, then pull off as it's not intermittent. Car stands while I work the rest of the event. Drive home, no issues.
This morning, driving at 65 or so, I hear and feel a 'hum', high frequency vibration, and as I accelerate it gets worse to be a lower frequency vibration in wheel and front end. Worse with brake. Pull off, decide to take car home...back on interstate, issue gone. Occasional squealing from left front. Squeal sometimes continues with no brake (squirrels in my front wheel). Intermittent. Sometimes stops with a little brake pressure. Sometimes car has slight pull to left.
Get back to garage. Jack car. Front left wheel won't spin at all on it's own. Right front a bit looser, but no free spin. (They were like that and it was noted by the guy that did the work, and in fact have been that way for some time, but getting progressively worse).
Stainless steel brake lines.
So, I suspect a caliper rebuild is in order? I'll do rotors at the same time.
Any suggestions for lighter weight rotors, and are cross drilled worth double the money for my application? What is a vented rotor?
On track I don't have sponginess issues, so not leaning brake cooling kits, but are those backing thingy's that purport to cool a worthy use of my capital?
Thanks
Scott

[notmytarga]

The wheels being hard to turn could be a number of things. Most should have been addressed by the work you had done. I like to blame myself and do all the work myself.

My first thought is wheel bearings were not adjusted or placed/lubed correctly. Bearing races not square, etc. Yes, it could be the caliper pistons, BUT I think you are blaming the mechanism that was not worked on - I would do the opposite. The new symptom is likely the result of an error in the work recently performed. The brake mechanism should get pushed away from the rotor with tolerable runout here and not cause much noticeable sound.

Removing the pads and caliper and checking the bearing feel will help separate these issues. Bringing it back to have them look at it could be an endless loop of denial. Again, another reason I do my own work. Don't invest in more parts until you know what is going on. You may need new rotors if the races are fused into the current ones.

[sbmackie]

Not my targa,
Thanks for that suggestion. I will pull them apart as you say. In defense of the tech that did the work, the brakes have been dragging since before I did the wheel bearings myself 4 eyars ago. It is not a new issue; the squeaking and occasional chattering are. The wheel bearings were inspected and adjusted by this tech.

Still, it will be instructive for me to pull the wheel apart.

Cheers

[sbmackie]

Drove car a bit, very easy on the brakes. Pulled wheels. Wheel centers (especially the left one, where all the squeaking is coming from) are HOT. Left wheel bound up pretty tight. Took pointy screw driver and pushed calipers back into place off discs. Very easy, little resistance. Wheels spin freely, no wobble or noise (bearings good and adjusted correctly, no wobble). Mash on brakes once. Still free, slightly more binding. Mash on brakes hard a few times and hold at "threshold braking" for a couple seconds. Brakes no binding again.
Rotor has darker mark at middle where I assume the pads are rubbing.
Right side is same, only less so. Rears are totally free.
With brakes sticking a bit, opened bleeder valve and relieved some pressure. Brakes still binding until I push them back.
So, my analysis is front calipers need rebuilding. I'll replace front and rear rotors, front and rear lines, and have an expert rebuild the calipers. Ill also get the valve thingy at the front of the master cylinder in case replacing the above doesn't work, which means then it is somewhere between the master cylinder and the caliper.
Thoughts?

[aston@ultrasw.c]

Originally Posted by sbmackie

I'll do rotors at the same time.
Any suggestions for lighter weight rotors, and are cross drilled worth double the money for my application? What is a vented rotor?

Stock rotors are vented and very good value, hard to beat.

Cross drilling, slots, cryo, etc is a waste of money.

[sbmackie]

Yep, I agree, especially for my AX/occasional light track and pleasure driving use.

What is the name of the little thingy (valve, diverter) that occasionally fails that is in front of the master cylinder that tells the fluid to go either front or rear? Or does the 911 master cylinder incorporate it?

Reason I ask is a non-Porsche type tech suggested I have one of those handy in case the caliper rebuild/line replacement/disc replacement doesn't solve the problem.

Thanks

[Bill Verburg]

Originally Posted by sbmackie

Yep, I agree, especially for my AX/occasional light track and pleasure driving use.

What is the name of the little thingy (valve, diverter) that occasionally fails that is in front of the master cylinder that tells the fluid to go either front or rear? Or does the 911 master cylinder incorporate it?

Reason I ask is a non-Porsche type tech suggested I have one of those handy in case the caliper rebuild/line replacement/disc replacement doesn't solve the problem.

Thanks

The m/c applies equal pressure to both the front and rear circuit at all times via a dual inline piston arrangement and dual ports(1 for front and 1 for rear)

from '84 there is an additional pressure limiting valve(aka p/v) on the rear circuit only,
this valve can be seen to the left(facing rear) of the m/c booster assembly, an '82 SC won't have it.

heat from the rotors could be caused by internally swollen lines or seals that don't allow the fluid to return to the m/c and reservoir

heat can also come from wheel bearing issues

all 911 rotors from '69 up are vented, stock for an Sc is 282.5x20mm front and 290x20mm rear w/ straight vanes separating the rotor face plates, 930 and most newer models have vented rotors w/ curved vanes that only work right on one side of the car, so there are left and right versions, The curved vanes are more efficient at moving cooling air through the rotors when they are on the correct side.

drilling is an additional step that most track guys try to avoid, the holes are where heat induced cracks propagate from.

If rules allow many change the SC fronts to 3.2 Carrera fronts which are hydraulically the same but have thicker 282.5 x24mm rotors

for A/X you are not likely to get heat induced cracks or any other sort of heat issues w/ stock SC brakes

juddering from brakes can have several sources
wheel bearings
worn shock bearings
pad deposition
worn rotors
etc

[911pcars]

Another place to look; restricted brake lines. The flex lines (factory or otherwise) can deteriorate internally and block fluid on either side of the embolism. If the rotor feels excessively hot, fluid pressure could be blocked on the caliper side.

Loosen a caliper bleeder screw and see if that makes a difference.

Sherwood

[ipacketeer]

Just a thought, if there is any air in the system due to above normal use of the brakes, generated heat expends the air which, would not allow the piston to retract, causing the problem you describe.

[sbmackie]

New rotors/caliper rebuild/new lines did the trick. Thanks for all the input