14 replies to this topic

[craigmcd]

Our family has had a sweet little 99 Outback for a number of years. It is well maintained, and received a new engine about 18 months ago. My daughter has had a recurring braking problem which has only gotten worse. -- When driving the car will begin to brake on its own, causing the rpm to rise suddenly and the brakes to become very hot. This is usually accompanied by some vibration in the steering wheel and floorboard. The brake pedal becomes stiffer (less toe play). The car does not suddenly come to a squealing stop, it just begins to brake entirely on its own - as if while driving along you would be holding the brake pedal 30 to 50% down while attempting to maintain speed. Earlier this week it happened to her again in town, shortly after leaving school (so the brakes were cool). Yesterday it finally happened while I was driving. While cruising at highway speed of 65 with 2500 rpm, I felt the vibrations, and the rpm jumped to 4000+. There were no significant sounds, the weather is quite cool, and I had not touched the brakes for several miles at least. The transmission may have downshifted with the jump in rpm, but I am not positive.

My mechanic looked at it a few weeks ago and lent me a heat sensor for us to read the pad backing and the disc (four wheel disc). I have been tracking normal, and my daughter the overheating. The normal temps for the front are between 160 and 180F. When there is an episode of overheating we are seeing temps of over 300F, with a number of reading exceeding 400F - so it is damn hot. We have gotten several readings of "Hi", which must be beyond the meter's ability to report accurately (I saw this myself). The car will drive normally again after a cool down period of 20 to 30 minutes.

--- Under normal circumstances the brakes work great, no pull, no vibration. New tires, newer alignment. I am a decent home mechanic, but I am stumped. Thanks for ideas ! CM

[john in KY]

A new brake master cylinder will fix this.

[Rooster2]

A new brake master cylinder will fix this.

Same thought........a new master cylinder. Years back, I had an Audi that did most of what you are describing. A new master cylinder fixed it.

New brake pads may also be needed.

[lostinthe202]

When this happens, reach down and physically pull up on the brake lever and see if it goes away or gets better, my money's on a new master as well.

Will-

[craigmcd]

Thanks for the thoughts. I understand about the brake pads. Is there any other component I should be worrying about as a result of the very high brake temperatures?

In more research someone mentioned the ABS wheel sensors - is that a possible cause? Craig

[Rooster2]

Thanks for the thoughts. I understand about the brake pads. Is there any other component I should be worrying about as a result of the very high brake temperatures?

In more research someone mentioned the ABS wheel sensors - is that a possible cause? Craig

I don't think the ABS wheel sensors is the cause. They never hold the brakes on. A sticking master cylinder that won't release brake pressure after you have depressed the brake pedal (stopping the car) is causing the problem.

The over heated brake pads may have warped the rotors. If you have a vibrating shutter when braking, it is prolly caused by warped rotors.

[Olnick]

My mechanic looked at it a few weeks ago and lent me a heat sensor . . . The normal temps for the front are between 160 and 180F. When there is an episode of overheating we are seeing temps of over 300F, with a number of reading exceeding 400F - so it is damn hot.

When you get the over-heating is it happening at all 4 wheels or just the fronts? Does one side get hotter than the other?

Have you checked your caliper slides and piston seals for "stickiness?"

[naru]

My money is on a faulty brake booster valve aplying the brakes w/o any pedal input.

Can`t see how a faulty master could apply brakes on its own.
Hold brakes on once applied-yes
Self apply brakes -no

I would disconnect the booster vacuum line,bleed off booster vacuum by pressing the pedal several times and go for a careful(no power braking) test drive as soon as possible after the problem occurs.

Or more simply:
jack a wheel off the ground
confirm that it is difficult to turn
bleed off the booster vacuum by removing the hose(thus leaving the master undisturbed)
see if the wheel turns more easily
If yes,booster is at fault

Edited by naru, 21 February 2011 - 08:03 PM.
better booster test

[john in KY]

Pads are always making light contact with the rotors. This contact generates heat that gets transferred to the brake fluid. Brake fluid absorbs the heat and expands as a result. The increased volume has to go somewhere and the somewhere is the master brake cylinder. If the heated/expanded fluid can not get back it will push the calipers pistons out. All this does is generated more heat/more expanded fluid until eventually the brakes will lock up. Been there, done that.

[naru]

OK,but if the heated/expanded fluid can`t make it past the master cylinder compenstion ports how did it do so after a brake application?

An inspection should easily reveal plugged compensation ports.
(fluid should squirt back into the resovoir upon brake release)

Might be worth checking booster pushrod clearance.

[craigmcd]

[quote name='Olnick']When you get the over-heating is it happening at all 4 wheels or just the fronts? Does one side get hotter than the other?

All four discs get very hot, but the right / rear is always a bit cooler than the others. By cooler I mean while the other reading are 350 to 400 on the disc, it will be 225 to 325F. The pads are always somewhat cooler, but I am reading the backing metal, not the pad itself. Great ideas, thanks again for feedback. Craig

[wally]

Can`t see how a faulty master could apply brakes on its own.
Hold brakes on once applied-yes
Self apply brakes -no

brake pedal has to be depressed to shift the car out of park, since it is an automatic. therefore, the brakes have been applied, albeit lightly and with the car not moving at the time.

[craigmcd]

Since this is an older car we have decided to replace both the booster and the master cylinder at the same time. To do just one or the other does not make a lot of sense because the labor is mostly done while replacing either component separately. We will also bleed out the old fluid and replace with new.

After a few days of driving I will inspect the brake pads and see if they should be replaced - they are not that old, but the overheating may have taken a toll.

Thanks again for the input, it is appreciated. Craig

[craigmcd]

This is a follow up message.

Several weeks ago we had the 99 Outback in the shop to fix this problem. We replaced only the master cylinder. The power booster was over $300 from Subaru, and there were no good aftermarket options available. NAPA could rebuild our core, but it would take over a week.

Anyway, the car has been driven city and highway miles pretty heavily, and the original problem (the brakes would activate all by themselves and the brakes would get very hot) seems to be completely fixed.

We also asked the shop to bleed out as much of the old brake fluid as possible. The brakes got very hot, and I am worried that the fluid may have overheated.

Thanks to those who made suggestions, I appreciate your help. Craig

[Olnick]

That's great news. Thanks for letting us know how it turned out.