924Board.org

[garthfan]

Hey all...more fun tales from the 924S.

Been having a problem with my brakes...the fronts lock up every time I drive more than a few miles. It's definitely both wheels locking up. The car doesn't pull to one side or the other when it happens. And -and this is important-, when the car sits for about twenty to thirty minutes the brakes just release on their own. So, last night I pulled apart the calipers, cleaned them up, reassembled and made sure they were operating properly and put on new pads to replace the metal things that I believe USED to be pads....very, very simple procedure on these cars, I might add. Bled the system just because and took it to work today.

Wouldn't you know it, on the way to work the front brakes locked up tighter than a virgin on prom night and by the time I had sense enough to pull over, the new pads on both sides were on FIRE!!! Crikey! After I put out the fires, I let the car sit for a few minutes and the brakes just magically released on their own -as usual.

So, now I'm confused. I have to find out why they're still locking up. Any ideas??? Is it possible for the brake fluid to cause some sort of a "hydro-lock" when it heats up??? The problem does make the pedal very, very hard to push. Or do you think this is simply a mechanical problem with the pistons??? It's interesting to me that they're BOTH locking up all the time so I kind of want to dismiss it as a mechanical problem with the calipers themselves. Especially after I've already worked on them.

Any help, as always is much appreciated.
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1987 924S - Was a daily driver...now it's GONE! *sniff*
1986 944 N/A Guards Red
1996 VW Jetta Grocery-Getter
1995 Saab 900S Winter-Driving-Feeling-of-Security-Vehicle

[Smoothie]

When they're locked up, try pulling up on the pedal - could be a pedal return spring problem. Other possibilities include bad spring/s inside the brake MC and/or the MC's sticking. Also could try opening a bleed valve while they're locked up to see if that releases them. If any of these things work, it's as you suspect not a piston caliper locking up problem.
look here- http://www.clubcobra.com/t45990.html
I suspect you have a master cylinder problem.
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[emoore924]

It does sound like it could be a sticky MC. What happens when you remove the vacuum source from the brake booster (a) when the problem is happening or (b) before you start out..., beware, you'll be on full manual brakes w/out the assist hooked up.

[Lizard]

ok this is a simple thing to fix, adjust your brake pedal end play, if your brake pedal doesnt have enough end play then it will close the opening/release to the brake fluid resevior, so as the fluid heats up it expands and the only piece which moves is the piston out and causes the lockup, as the brake fluid cools it releaves the pressure and releases the brakes and you can drive again until it heats up and expands,

I would recommend also flushing the fluid as it has probably been burnt, I also recommend taking apart the calipers and looking at the piston if they have heat discolouration then you need to replace the piston or entire caliper.

good luck with it,
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3 928s,

[Neil924]

I saw the CGT brakes catch fire a couple of times. {Way too cool for school!}

[kaffine]

I haven't seen brakes catch fire before but I've seen the aftermath at work. A Freightliner semi truck completly burnt to the ground because the brakes caught fire.

I would be looking at brake pedal free play, binding linkage, booster, or master cylinder to cause the problem you described.
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80 924
80 931

The best desciption of an atom boils down to something unknown is doing we don't know what.
Sir Arthur Eddington

[garthfan]

As always, thanks for the input.

After investigating the problem further, I found that -as suggested- I could in fact crack the bleeder screw on just one wheel and the brakes would release. And it doesn't matter which side I crack, either. So obviously I'm building up pressure in the line going TO the calipers, but it's not being allowed to flow back into the MC.

So, I ordered a new master cylinder and I'm putting it in tonight. I did try to adjust the pedal, but it was set fine to begin with.

My question is this: Do I need to be concerned about any other component of the system possibly being at fault??? I'd like to have a couple ideas before I get into it too deep. I don't mind admitting that I'm not that good with brakes...I've just never had to deal with them too much.

Also, any tricks I need to be aware of before yanking the MC??? Thanks a bunch, guys!
_________________
1987 924S - Was a daily driver...now it's GONE! *sniff*
1986 944 N/A Guards Red
1996 VW Jetta Grocery-Getter
1995 Saab 900S Winter-Driving-Feeling-of-Security-Vehicle

[emoore924]

Does sound like it might be m/c but there are times when the inside of the brake line degrades and a little flap of old rubber stuff can block the movement of the brake fluid -- Also, just a reminder to check the brake operation with the vacuum assist disabled (in your driveway) before you start ripping things out.

Tips:

Once removed, check to old m/c to be sure it is your problem. It may compress (watch for squirting brake fluid) but you want it to *not* return cleanly if it is your problem.

Don't get any brake fluid on the paintwork. Best paint remover I've ever seen. Removes paint darn near instantly. Cover your fender, the engine compartment, inner fenders...

A turkey baster works well to remove old brake fluid from the resevoir before removing the m/c from the car. Helps avoid splashing.

It is likely that after m/c replacement the pedal travel will be set fine but check travel per haynes.

Be sure your old m/c has the same kind of brakelight switching as the new one -- some had a pressure switch in the m/c, some were switched inside the car at the pedal. A dead giveaway is when the brake lines don't seem to match the fittings on the new m/c or you have wires left over.

Some m/c's need to be pre-filled and bled. Read the instructions. Be careful to NOT inadvertently push/jam/shove the m/c pushrod/piston all the way to the end of its travel (like if you step on the pedal with no fluid or no pressure in the system). There are little rubber and metal parts in there that like the bore to be the right diameter and jamming the assembly at the end of the travel can goof that up.

Once m/c installed, bleed out system starting with the longest brake line first -- i.e. right rear, left rear, right front, left front. To do it right, I would have two quarts of dot 4 available -- you'll probably use a little more than 1 to do the whole job including topping off the resivoir when you're done. Keep an eye on the resevoir as you don't want to run dry or you'll need to start all over again.

Use brake fluid from a new, unopened container. I would suggest dot 4.

Be sure to pump the brakes a few times to ensure the pads are set in the calipers properly or you might have a short "no brakes" experience if you've messed with the pads/pistons. This can be a problem if a wall, vehicle or other immovable object enters the picture before the experience has been resolved.