Mechanics

BRAKES COME ON ALL BY THEMSELVES

2004 Mazda MPV • 128,000 miles

Our 2004 MPV Van recently began applying its own brakes. After being driven for about 5 minutes it would begin to need more and more gas pedal to maintain speed. It did not take long to realize that the car was braking since lifting the right foot caused rapid deceleration and then a complete stop. The problem can pass, or ease up, after a while, and return again, but it is not safe to drive like this and it will surely ruin the pads/rotors.
When testing the car to see if everything is moving freely before taking off I will place it in Drive and let off the brake pedal. If it moves without applying the accelerator it is not grabbing. I will even test again by moving it to Neutral and watching the speed as indicated. If there is not a drop in velocity it is still moving freely. The brakes/calipers will close with no pressure on the pedal. As an example I have tried to drive it to a mechanic and while on the freeway done the Neutral test and then resumed driving (never touching the brake pedal) and the shortly the brakes will apply themselves.

I have had the local Firest*ne shop look at it three times and Mazda dealer twice. In order the responses were:

FS) --- there is no problem.
Mazda) --- there is no problem.
Mazda) --- front caliper sticking. (Both calipers replaced)
FS) --- I don't see a problem but maybe your rear pads are sooo worn that the piston/pistons are far enough out that the cylinder lip is catching and not allowing them to retract or bad/damaged hose. (New rear calipers and pads were installed)
FS) --- It is the ABS pump. (ABS fuse pulled to remove ABS module from the equation)

Problem continues. Firestone was used for three reasons; the problem occured nearest to them, they were open and they were aware of the problem. Have not taken the next step as I don't relish the thought of spending $1,200 plus labor on another guess. The only parts not replaced seem to be the ABS pump, brake booster, the master cylinder and hoses.

The vehicle does stop in a straight line. All four calipers are engaging and not just a little bit. If you were to let off the gas at freeway speeds it would drop to 40mph almost instantly. The van will stop itself, and quite rapidly. Rotors have never been red hot but do vaporize water instantly. It is worth noting that I am deliberately not applying the brake pedal unless absolutely necessary and when it happens on the freeway I have not touched them at all.

It isn't the rotors or the parking brake. I checked that when I found out it has rotor/drum combination on the rear wheels. While investigating that was when it was noted that all four brakes are engaging at the same time.

Rotors are straight (okay circular) and within specs. There is no vibration or pulse when applying the brake.

The pedal is hard and not spongy or soft but I honestly don't know if there is any pedal movement b/c if there is I didn't see or notice it.

Any insight would be greatly appreciated.
Avatar
Doublesharp
May 19, 2012.




First of all, note which wheels get hot, if it's one, two, or all four. The next time they apply themselves, crawl underneath and open one of the bleeder screws You can also loosen the steel lines at the master cylinder. If opening any one bleeder screw or line lets all of the brakes release, the most likely cause is the brake fluid is contaminated with some petroleum product such as engine oil, power steering fluid, or even axle grease. The additional clue is the rubber bladder seal under the reservoir cap will be blown up and mushy. If only one wheel is dragging, suspect the rubber flex hose. You can identify that by the brake releasing when you open the bleeder screw but not the steel line at the master cylinder.

Caradiodoc
May 19, 2012.
I haven't heard that of that test yet. I will try that tomorrow. Thank you for the tip.
All four get hot. Tonight I took it for a drive to see how long until it occured and it was almost 10 minutes. Got home (barely) and quickly grabbed my IR thermometer.648 front high temp and 362 rear high temp.
While driving I was constantly pulling up on the brake pedal with the top of my foot to be sure the pedal was returning to its proper height. There was no sag in the pedal placement. It was returning the up position.
AD

Tiny
Doublesharp
May 19, 2012.
I don't like giving this diagnosis but it sounds like the brake fluid is contaminated. The very tiniest amount of petroleum product will cause all rubber parts to swell. The lip seals on the pistons in the master cylinder grow past the fluid return ports and block them. That keeps the fluid trapped in the system where it keeps a little pressure on the calipers. As the dragging brakes heat up, that heat is transferred to the brake fluid which heats up and expands. Since it can't get back into the reservoir, the pressure applies the brakes harder, and they get hotter. It's a vicious circle.

To prove this, when the brakes are dragging, loosen the two steel lines at the master cylinder. If you see a little spurt of fluid and the brakes release, fluid contamination is the problem. The only proper fix for that is to replace every part that has rubber parts, and flush and dry the steel lines. The anti-lock brake hydraulic controller must be replaced because it has rubber seals. Also the calipers, wheel cylinders, combination valve or height-sensing proportioning valve, master cylinder, and all of the rubber hoses. I know the calipers have been replaced already but they are contaminated too. All rubber parts must be replaced at the same time, otherwise the contaminant will leach out of the old parts and into the new ones.

If you can figure out who worked on the vehicle just before this started happening, you might get them to cover the repairs. Two things that used to be common were wiping out a funnel used for engine oil or transmission fluid, then using it to fill a hard-to-reach master cylinder or a brake bleeder ball, and repacking the old-style tapered front wheel bearings, wiping off your hands, then using your fingers to poke the rubber seal back into the reservoir cap. The residue left on fingers or that funnel was enough to cause problems. If the reservoir cap was left off while someone hunted for a bottle of brake fluid, and it fell down onto the engine that had some leaked oil on it, that too would cause a problem.

Be sure to check that bladder seal under the reservoir cap. If it is mushy and grown too big, you will have to fight like crazy to get it back in the cap. They normally pull down over time as the brake fluid leaves to fill in behind the caliper pistons as the pads wear down, but a simple poke is all that should be needed to pop it back into the cap.

Caradiodoc
May 19, 2012.
Great info! Thanks, Caradiodoc.

Tiny
Shawej
Aug 1, 2014.

AD