Rear brake pads caught on fire!

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?

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

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.

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.

SeanR;4805 wrote: ...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.

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