Rear brake pads caught on fire!

More
01 May 2011 13:43 #4649 by masterchan
Hi, fellow 928 owners. I would like to pick you guys' brain on what might have caused my rear brakes to overheat so badly.

I didn't even think it's possible, but yesterday the rear pads caught on fire as I was coming out of the highway. It was a small one and I blew it out myself (yes, with my mouth - it must have been a pretty hilarious sight… for other people at least) :(

My 85' 928 Manual has been having problem with the rear brakes getting really hot. It all started when I got new rear rotors and pads. Five months after that, I noticed some burnt brake smell and brought it back to the shop. The shop told me the aftermarket pads they put in weren't holding up and replaced them with OEM ones. And then about a month later, one day as I was coming off the highway, I started to smell burnt brakes and noticed smoke coming out of my rear wheels!

I took the car to a different shop, who rebuild the calipers, replaced the rotors and pads, and had the brake line redone. A few days after that, same problem occurred - brakes got really hot as I was coming off the highway, but there was no smoke yet. Took the car back to the shop. They did a bunch of tests and concluded that there wasn't anything wrong - the temperatures of the front and rear brakes are the same in the test scenarios they put the car through.

And then, yesterday, the rear pads smoked up and one of them even caught on fire.

I'm 100% positive I did not leave the parking brake on in all those cases. I also wasn't driving the car hard at all - in all those cases I was slowing the car down from 70 miles down to about 30 or so. I'm not even that frustrated with the shop's inability to get it right - I just want to get to the bottom of this and not have smoke coming out of my wheels… brake smell just isn't the most pleasant smell the world has to offer.

Any idea what's going on here?

Please Log in or Create an account to join the conversation.

More
01 May 2011 14:33 #4650 by 928mac
Hmmm, Chan,
I am not sure what to tell you as you did everything right by taking it back to the shop and had them test every thing.

Did this start right after a repair was done or did it happen slowly over time?

For a pad to catch fire it has to be extremely hot and over heated or there needs to be oil on it.

BTW if they got that hot then you will have melted the grease in the wheel bearings and you should be careful.

If it was me, I would jack the back of the car up and remove the wheels.
then see if the rotors turn free by hand. (they should)

Then pump the brake and turn them by hand again. They should be completely free after 1 turn.

If after the brakes are pumped and you can not turn the rotor by hand, wait 1/2 an hour and try turning the roter again (do not pump).
If it frees up by just sitting then it is a restriction in the line, a collapsed flex hose, or a stuck valve or bleed back in the Proportioning valve - ABS system or the master.

The way to check this is to eliminate the back of the car as the problem by cracking the rear line at the front of the car just before it goes into the front of the ABS unit.

It looks like the top frt port on my car (89) and it threads into a large round valve attached to the ABS unit.

I hope this helps give you a place to start.

Let us know how it goes.

Brad

This image is hidden for guests.
Please log in or register to see it.


Please Log in or Create an account to join the conversation.

More
01 May 2011 21:57 #4657 by pcar928fan
Agree with Brad about the wheel bearing grease and it might well have been that catching on fire or heating up so much it is catching the brakes on fire. I have NEVER heard of this happening before...

If it was not bad wheel bearings to start with I suspect they are bad by now almost assuredly.

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

Please Log in or Create an account to join the conversation.

More
02 May 2011 08:58 #4666 by SeanR
There is no "wheel bearing grease" in our rears. They are an enclosed/sealed bearing and you would know if they were bad. Grinding/humming sound at slow speed that progressively get higher in pitch.

As to the brakes, I'd check first to see how tight the emergency brake is. To do that, lift the rear of the car up (jack from center of rear cross member) then try and rotate the tires by hand, both sides. If this is good then move on to the braking system.

First off, do you have ABS on the car? Not that it really matters, but takes out the brake bias valve that Brad has pictured.

Check that first and then post up what you find and we can go from there.

Please Log in or Create an account to join the conversation.

More
02 May 2011 09:36 #4667 by martinss

...As to the brakes, I'd check first to see how tight the emergency brake is.


I had similar issues many years ago when the parking brake would seize due to rust. It did not do that all the time, only sometimes - so it was hard to diagnose. I don't know enough about 928 parking brakes to know if this is possible.

Please Log in or Create an account to join the conversation.

More
02 May 2011 11:17 #4668 by 928mac
Thanks Sean, I was hoping that you would get involved.
I have not had the rears apart yet so I can only talk strategy and not experience.
Dose the 928 have a drum e-brake or is it caliper actuated

Brad

Please Log in or Create an account to join the conversation.

More
02 May 2011 11:38 #4669 by srshaw3

Thanks Sean, I was hoping that you would get involved.
I have not had the rears apart yet so I can only talk strategy and not experience.
Dose the 928 have a drum e-brake or is it caliper actuated

Brad


drum e-brake

Please Log in or Create an account to join the conversation.

More
02 May 2011 12:10 #4670 by SeanR
Yup, as Stan said, drum.

In case anyone else searches looking for e-brake adjustment, the adjustment for it is behind the e-brake handle. You access it by taking the cover off, 4 screws hold it in place. Two up top, two on the side facing the inside of the car. From there you will see the cable as it enters the body and attaches to the handle assembly via a spring and a set of bolts. Tightening the cable (pulling it further in to the car) tightens the brakes.

A properly adjusted e-brake will only take 3 clicks, when pulling up, to hold the car in place.

Please Log in or Create an account to join the conversation.

More
02 May 2011 12:42 #4671 by srshaw3
I always think the e-brake handle should be adjusted so it isn't any higher than the height of the seat base, as otherwise getting in and out of the car with it engaged could be damaging ..... and not to the car :)

Please Log in or Create an account to join the conversation.

More
02 May 2011 13:12 #4672 by 928mac
So this means that the e-brake will have nothing to do with the brake cal/pads catching fire, right?

Please Log in or Create an account to join the conversation.

More
02 May 2011 13:50 #4674 by srshaw3
I would expect that to be true. Perhaps the only exception I can think of, would be if the e-brake caused excessive heat to the whole axle hub assembly, but that may be far fetched, and I would expect you would know whether that is possible more than I.

Please Log in or Create an account to join the conversation.

More
02 May 2011 13:57 #4675 by srshaw3
Maybe I missed it, but are we certain it is the pads catching fire, as opposed to some foreign matter? CV boot grease, mice bedding, etc.

There is an adjustment on the brake pedal that can cause it to put pressure on the brake unintentionally. A 928 racer (not me) made an adjustment and after a few laps on the track the car would stall when the clutch was released. Apparently he did this in two separate track sessions, where he had to be towed back, the second time flat bedded as the wheels would not release. As I recall the pedal/master cylinder adjustment can be made so that the brakes do no release as they should, so the brakes are always on....

Please Log in or Create an account to join the conversation.

More
02 May 2011 14:48 #4676 by 928mac

Maybe I missed it, but are we certain it is the pads catching fire, as opposed to some foreign matter? CV boot grease, mice bedding, etc.

There is an adjustment on the brake pedal that can cause it to put pressure on the brake unintentionally. A 928 racer (not me) made an adjustment and after a few laps on the track the car would stall when the clutch was released. Apparently he did this in two separate track sessions, where he had to be towed back, the second time flat bedded as the wheels would not release. As I recall the pedal/master cylinder adjustment can be made so that the brakes do no release as they should, so the brakes are always on....


This is awesome information Stan that I will try and research.
Thanks, It would not surprise me that there is an adjustment like that on the Porsche.
I suspect that it would be a C-click notch adjustment on the back of the master but it could be on the peddle side.

Please Log in or Create an account to join the conversation.

More
02 May 2011 15:12 #4678 by srshaw3
I emailed the driver who knows, and will let you know what I hear back. Do you think this could be related to this issue?

Please Log in or Create an account to join the conversation.

More
02 May 2011 16:31 #4680 by 928mac

I emailed the driver who knows, and will let you know what I hear back. Do you think this could be related to this issue?


There are to many possabilities, but I don't think it would move on its own.

I have even seen where a mechanic squashed a brake line on a north Am. car by lifting it wrong on a hoist.

I have seen brake hoses that failed inside and would only flow one direction.

That valve I pointed to in the picture could stick intermitently.

Sometimes these things are hard to find if everything is working when you are testing it.

Sometimes it requires a long hour or two drive to get it to fail so that you can fix it.

We'll see what he finds.

Please Log in or Create an account to join the conversation.

More
02 May 2011 16:48 #4681 by masterchan
Thanks for all the replies!

So I took the car to the shop and they were able to reproduce the problem on the rack.

After they ran the car on the rack for about 45 minutes, the rear brakes started to engage on their own. They pinpointed the problem to the master cylinder getting hot and expanding. This is why the problem always occurred after I've been driving on the highway for a while. The brakes were dragging on fast rotating wheels for long periods of time. It just so happened that I was lucky to be coming out of the highway in all those cases.

If anything, I think this might be a pretty unique and interesting case study. This is actually a pretty reputable shop and the guys working on my car have had 20+ years of experience. Both of them said they have never seen anything like this before.

-Julian

Please Log in or Create an account to join the conversation.

More
02 May 2011 17:14 #4682 by SeanR
Bingo, this reminded me of an '87 that we had this issue with a few years ago. Totally forgot about that. His were actually locking up after 10 minutes or so of driving. They never had the chance to get that hot.

A small amount of corrosion in these things can cause a bunch of different issues.


Thanks for all the replies!

So I took the car to the shop and they were able to reproduce the problem on the rack.

After they ran the car on the rack for about 45 minutes, the rear brakes started to engage on their own. They pinpointed the problem to the master cylinder getting hot and expanding. This is why the problem always occurred after I've been driving on the highway for a while. The brakes were dragging on fast rotating wheels for long periods of time. It just so happened that I was lucky to be coming out of the highway in all those cases.

If anything, I think this might be a pretty unique and interesting case study. This is actually a pretty reputable shop and the guys working on my car have had 20+ years of experience. Both of them said they have never seen anything like this before.

-Julian

Please Log in or Create an account to join the conversation.

More
02 May 2011 17:28 #4683 by pcar928fan
So Sean what is the answer to fix this problem??? I had never heard of this up till now...and certainly never heard of pads actually catching FIRE! OMG! That would suck...though not as bad as the engine fires when a fuel hose cracks all the way through...

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

Please Log in or Create an account to join the conversation.

More
02 May 2011 19:04 #4684 by srshaw3
I heard back from the 928 racer who had the issue, he said:
"could be master cylinder related or mal-adjustment of the free play in the brake pedal. That would affect front and rear, not just rear. Brake piston doesn’t withdraw past the pressure relief hole in the MC. Every application of the brakes builds more pressure till all 4 stay “on”.

Please Log in or Create an account to join the conversation.

More
02 May 2011 19:52 #4685 by 928mac
Well it sounds like we were all wrong.
I would like to know why the master is heating up and is it just a matter of it being over full or the cap not venting?

Please Log in or Create an account to join the conversation.

More
03 May 2011 08:42 #4687 by SeanR
James, this was on Dannyfumi's '87. We'd replaced dang near everything and done multiple bleeds to no avail. We decided to replace the master cylinder because it was the only thing not new, and bingo, it worked.

Please Log in or Create an account to join the conversation.

More
03 May 2011 11:48 #4691 by pcar928fan
WOW...that is crazy! Like you I would never have thought it was that MC...like you I would have replaced everything else and that would have been last. At least my "logic" (as suspect as it is) matches up with a pro! I hope I NEVER see anything like that with ANY of my cars!

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

Please Log in or Create an account to join the conversation.

More
05 May 2011 13:51 #4711 by masterchan
In conclusion...

Got the Master Cylinder replaced and now the rear brakes seem to be fine - no more spontaneous phantom rear braking. Man, this whole thing was such a drag :)

Julian

Please Log in or Create an account to join the conversation.

More
05 May 2011 15:16 #4715 by 928mac

In conclusion...

Got the Master Cylinder replaced and now the rear brakes seem to be fine - no more spontaneous phantom rear braking. Man, this whole thing was such a drag :)

Julian



ROTFLMAO

I am glad you found the problem.
We need to document this somewhere Stan so that others can view the info.
What do you think?

Please Log in or Create an account to join the conversation.

Moderators: Rich928mm928billvvOC

Don't have an account yet? Register Now!