Brakes Applying all by themselves??

[MACP34]

Hey all...ive been plagued by a phantom braking problem thats becoming really dangerous. While driving, the brakes will all of a sudden come on all by themselves without me putting ANY pressure on the brake pedal. What is THAT all about? Today, i had the Explorer almost come to a crawl on the highway and i could barely get it to the side of the road with FULL throttle. For some reason, the brakes were stuck on but i hadnt ever applied the brakes. Ive replaced the calipers, the pads (twice so far and now im sure i need to do it again since they were roasted twice today), the Master cylinder has been done twice by Ford. Im at a loss whats causing this. Could it be a brake booster with a vacuum leak or something? Any help would be greatly appreciated.

Mike

 

[BrooklynBay]

Do you get any brake, or ABS lights coming on? Do you feel any pulsating from the ABS system? Did you check your parking brake mechanism?

 

[MACP34]

E-Brake is free and clear. No warning lights on the dash. No ABS pulsing. Its like the brakes just apply themselves (and pretty hard too).

 

[BrooklynBay]

When this happens, does the brake pedal actually go down, or do the calipers just apply somehow? Did you ever check to see if the pedal goes up when you shut off the engine?

 

[MACP34]

THe pedal doesnt go down when this happens. And when i shut the engine off, it doesnt necessarily relieve the pressure (at least right away). I do notice that when this happens, there is NO play in the brake pedal. Just breathe on it and the brakes are right there. Theres not the usual brake travel even though the pedal isnt being pushed down.

 

[gijoecam]

I'd put my money on a bad booster myself...

-Joe

 

[dogfriend]

Push rod adjustment?

 

[gijoecam]

I considered that, but ordinarily, the push-rod being out of adjustment would cause the brakes to drag, but not to spontaneously apply themselves. A bad valve (not the check valve) in the booster, however, could cause the symptoms.

-Joe

 

[briwayjones]

If he's replaced all of those components could it be a problem with the ABS computer itself since it seems to have a mind of it's own just coming on and not really hanging up when the petal is actually pressed?

 

[MACP34]

well to eliminate the brake booster as the culprit, Ford disconnected it from the master cylinder and drove it. Once again, the brakes applied themselves. They released the pressure at the master and the calipers released. So they believe its definitely a bad master cylinder. I told them that the odds of 3 master cylinders exihibiting the SAME problem are astronomical. He menitoned i should go buy a lottery ticket tonight. I would tend to agree! I told them to road test it beyond belief because i cant handle the horror of it coming to an agressive halt in the left lane of the highway again. The nice part is the repair is under warranty. The bad part is im sure i roasted the new brake pads. And the fun continues...i'll post updates as i get them.

 

[gijoecam]

well to eliminate the brake booster as the culprit, Ford disconnected it from the master cylinder and drove it. Once again, the brakes applied themselves. They released the pressure at the master and the calipers released. So they believe its definitely a bad master cylinder. I told them that the odds of 3 master cylinders exihibiting the SAME problem are astronomical. He menitoned i should go buy a lottery ticket tonight. I would tend to agree! I told them to road test it beyond belief because i cant handle the horror of it coming to an agressive halt in the left lane of the highway again. The nice part is the repair is under warranty. The bad part is im sure i roasted the new brake pads. And the fun continues...i'll post updates as i get them.

Now I'm confused.... driving down the road, foot off the pedal, brake booster vacuum hose disconnected, and the brakes applied themselves? And than, to relieve the pressure, they cracked a fitting at the master cylinder and they released? There's only two things that can generate pressure in the brake lines: The master cylinder push-rod being depressed, or the pistons in the caliper(s) being pushed back into the calipers with the master cylinder piston being back past the compensating port. The only two things that can push on the master cylinder push-rod are the pedal and the booster.

Now, this has got me thinkin' about the ABS.... Maybe, somehow, one of the valves is (incorrectly) opening and the ABS motor is releasing some stored pressure into the lines. It would require a catastrophic mis-timing of the valves, because ordinarily that pressure should bleed back to the master cylinder, but I suppose the blocking valve could energize at the same time as the pressure..... If it's malfunctioning, I guess anything is possible.... but that wouldn't explain why cracking a line at the master cylinder would release the pressure?? And, any uncommanded malfunctions like that *should* trigger an ABS light and disable the system.

Eliminating the ABS system from the equation is as simple as pulling the fuse. Can't hurt to try it.

I've still got my money on the booster.

-Joe

 

[mildmechanic]

well to eliminate the brake booster as the culprit, Ford disconnected it from the master cylinder and drove it. Once again, the brakes applied themselves. They released the pressure at the master and the calipers released. So they believe its definitely a bad master cylinder. I told them that the odds of 3 master cylinders exihibiting the SAME problem are astronomical. He menitoned i should go buy a lottery ticket tonight. I would tend to agree! I told them to road test it beyond belief because i cant handle the horror of it coming to an agressive halt in the left lane of the highway again. The nice part is the repair is under warranty. The bad part is im sure i roasted the new brake pads. And the fun continues...i'll post updates as i get them.

Did you ever figure this out? I'm having the same issue with an older explorer with low mileage. Changed the booster and caliper and still having an issue...

 

[Rick]

Did you ever figure this out? I'm having the same issue with an older explorer with low mileage. Changed the booster and caliper and still having an issue...

My best guess is that you have a bad soft brake line. The insides can deteriorate to the point that they will not allow fluid to flow freely. I have heard of wheels locking up when a rubber hose goes bad internally.

 

[CDW6212R]

Ditto, the hoses can restrict fluid flow enough, that it traps some pressure in the caliper side. At this age for any braking issue like that, replace the calipers and their rubber hoses, in pairs. If the problem seems to be the front, do both front calipers and hoses. Same for the rear, do them in pairs because any problem like that is more prone to be in both sides to some degree. Long storage without driving is a big cause for calipers to stick or drag.

Also, be wary of rebuilt calipers from parts stores. Watch the brake function carefully after those, old cores can have issues they didn't find during rebuilding them. I replaced the rear two of my black 98, and about nine months later I decided the pads were still wearing too much on one side. I got them to replace it again, and the pads then lasted twice as long.