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Slam · Posted: Thu Aug 17, 2006 1:53 pm Post subject: Burned brakes

Mandy (Gracie - my wife) came home tonight with her brake pads smoking like crazy. I had put in a new master cylinder and adjusted the rear brakes frequently over the last couple of days. Car's been bled twice and will again tonight. Her greatest concern (and mine) is damage to the rotors, bearings, calipers or pistons. The rotors have discolored and are now blued. I did this on my 944 and it was fine, but you guys know these 924 cars better than I do. I did notice the rear brake lights were still on after the car had come to a stop, so I'm assuming air in the system had heated and expanded, locking up the front rotors. Rears were warm but not smoking. Fronts smoked. Should I sand the front pads? Are the rotors toast (so to speak)? Hot air was hissing out the grease cap on the spindle - must have been quite hot. Is the bearing grease toast too? Can I just re-bleed and keep an eye on things? I know brakes are subject to considerable heat and maybe I'm being a little too worried, but they are, after all, brakes. Crucial. The car's grounded till further notice.

Thanks all in advance.
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'84 944 - kid blew motor
'83 944 - resting comfortably. For 12 years
'87 944 - sideswiped by trucker
'80 924 - gone
'78 924 - gone
'77 924 - rusting comfortably

Mike924 · Joined: 12 Aug 2004 Posts: 2601 Location: IoW UK

Once the discs have blued they're only fit for the bin.

The pads will be glazed which, yes, can be removed by sanding, but why bother if you're going to have to replace the rotors. You might as well get new pads too.

This sounds like a major overheat, so you'll certainly want to replace the grease in the bearings.

Heat build-up of the proportions you've described is well outside the normal operating conditions, so you have a definite problem somewhere which needs to be traced and fixed. It's no good hoping this one will just go away...sorry.

Is the new M/C operating correctly? Especially the return spring...? I'm sure your wife knows better than to 'ride' the brake pedal.
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1993 Porsche 968 Coupe, Midnight Blue, 6 spd

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OutOfTheBox · Joined: 31 Mar 2006 Posts: 434

Ozzie · Posted: Thu Aug 17, 2006 9:56 pm Post subject:

On the early models the brake lights operated off the MC.
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924RACR · Posted: Thu Aug 17, 2006 10:18 pm Post subject:

I'd be wondering if the brake MC is no good. You shouldn't have any retained pressure in the fronts like that. The only other possibility that comes to mind might be a stuck caliper - but that would be unlikely to happen on both sides! Still, worth investigation.

As for what to repair/replace - front rotors and pads both, no question, and I'd consider replacing the front wheel bearings themselves, not just the grease. They're cheap enough. Also consider rebuilt fronts (or rebuild them yourself - a lot cheaper, not that hard).
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gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

I would guess that either your "new" master cylinder is no good (was it really new, and the correct part for your 924?) OR, you didn't adjust the vacuum booster pushrod length after you installed the new master cylinder, and as a result, the brakes were partially applied all the time, causing the overheating.

Slam · Posted: Fri Aug 18, 2006 5:02 am Post subject:

Thanks, guys.

For the most part, you've confirmed what I suspected. I did bleed the brakes again last night and got one or two small bubbles out of the RR line. I also noticed more play in the pass. side front wheel bearings. The grease must have cooked out of them. I know the play increased because I checked them last week and they were fine. I got the master cylinder from EBS - they haven't sent me the wrong stuff so far, and the one I removed was indentical to the one that went in. The brake lights operate on a pressure switch on the MC itself. I did have some pedal freeplay when I put the new one in - perhaps not enough. The factory paint mark was undisturbed, so I left the pedal alone. I know better, but I can't recall Haynes mentioning adjusting freeplay. At any rate, my mistake. Don't think the calipers stuck - both sides at once? I'll be (or she'll be) putting in new pads, rotors and bearings. Better now than in January...

As Artie Fufkin said in Spinal Tap - "Kick this ass. Go on. Kick it."

Again, thanks, folks.
_________________
'84 944 - kid blew motor
'83 944 - resting comfortably. For 12 years
'87 944 - sideswiped by trucker
'80 924 - gone
'78 924 - gone
'77 924 - rusting comfortably

Lizard · Posted: Fri Aug 18, 2006 5:44 am Post subject:

you will not be able to rebuild those calipers if the rotor is blue you will want to get a rebuilt and give them yours as core unit, the reason being is that the piston will have gotten blue as well and the coating on it will start to peel and will shred the rubber seals,

I have to agree with gohim on this that the brake pedal freeplay was not checked and or adjusted and that this is the cause.
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Slam · Posted: Sat Aug 19, 2006 3:16 am Post subject:

I checked pedal freeplay - it was fine after all. Culprit is/was stuck caliper. Must have dislodged some rust or debris when bleeding. I'll do the calipers and go from there, but I agree that it'll probably need a front brake redo.

Again, thanks guys.
_________________
'84 944 - kid blew motor
'83 944 - resting comfortably. For 12 years
'87 944 - sideswiped by trucker
'80 924 - gone
'78 924 - gone
'77 924 - rusting comfortably

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

A stuck caliper will not cause both front calipers to lock, as the 924 has diagonal braking circuits.

A stuck front caliper will not cause the brake warning light to illuminate.

The stuck caliper could be the result of overheating, and not the cause, or the result of forcing the caliper piston to retract into the collection of sludge and crud that accumulates at the bottom of the caliper cylinders in the course of normal use when fitting replacement brake pads.

I would look for another problem.

ic932 · Joined: 11 Feb 2005 Posts: 1104 Location: UK

when were the flexible rubber hoses changed last? these can perish internally. this prevents hyraulic pressure being released from the calipers. feels identical to calipers binding....although for it to happen on both sides at the same time would seem unlucky to say the least.

968rz · Joined: 21 Aug 2006 Posts: 537 Location: S. E. Wisconsin

What about adjusting the pedal itself? I was "playing" with my 79 once and adjusted the pedal to get the hardest feel I could (the adjustment is at the pedal under the dash), drove about 5mile and my front brake rotors were glowing. If the MC is differant or mod'd in some way this could be the cause, No?
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Rick
79 924 coupe Petrol Blue 3spd auto (wife's DD)
93 968 coupe Amazon Green 4spd Tip (my DD)