Brakes intermittently locking up

Tiny
ETHER727
  • MEMBER
  • 2000 FORD TAURUS
  • 3.0L
  • V6
  • 2WD
  • AUTOMATIC
  • 197,000 MILES
My car's brakes locked up on me a few months back, the brake pedal was hard to push. By the time the tow truck picked up my car, the brakes were working fine again. The mechanic replaced the front rotors and brake pads. The car worked fine for a while and then the brakes started engaging and the pedal got harder. I looked up the problems related to the power brake booster, check valve, and ABS and believed the system was related to the check valve. I had the mechanic replace the power brake booster with the check valve and the problem has become more frequent since having it replaced. I checked for vacuum leaks with started fluid and saw no change in idle. I also disabled the ABS by removing the fuses even though the problems described did not relate to my problem. Any help in isolating the problem would be greatly appreciated. Sincerely, Paul
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Friday, May 24th, 2019 AT 1:33 PM

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Tiny
CARADIODOC
  • MECHANIC
  • 33,699 POSTS
The goal now is to get the problem to occur, then stop on a slight incline, shift to neutral, place a block about a foot downhill of one tire, then open the hydraulic system at various places to figure out where the brake fluid is being trapped. When you let the pressurized fluid release back to the reservoir, the brakes will free up.

You can start by loosening the steel lines right at the master cylinder, but chances are that's not going to do it. Next, you'll need to crawl underneath and open the bleeder screw on the caliper. Feel for the front wheel that is really hot from the dragging brake, and start with that caliper. If opening the bleeder screw lets the brake release, the typical cause is a constricted rubber flex hose. The second clue is the high and had brake pedal you observed. You have to push harder than normal on the brake pedal to force brake fluid through the restriction, then the fluid can't return to the reservoir when you release the pedal. As that brake gets hot, the heat migrates into the brake fluid and causes it to heat up and expand, and that makes that brake apply even harder.

If the hose proves to be constricted, suspect rust has built up inside the metal bracket where it is crimped around the hose. That's at my nifty red arrow in the photo. Use a large flat blade screwdriver, or possibly a large Channel Lock pliers to open that crimp up a little.
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Monday, April 5th, 2021 AT 12:21 PM
Tiny
ETHER727
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  • 14 POSTS
Thanks, I was unable to create the problem and make it to my driveway with the problem in tack. The problem mostly occurs while on a main road/highway driving 40 mph plus without even touching the brakes. Further, I forgot to mention, both front brakes are locking up.
Due to the age of the hoses and the way they looked, I replaced them and while the hose was disconnected from the caliper bleed them to make sure there was no debris. I also bleed them after connecting them to the caliper. Would this indicate there is no blockage from the master cylinder and ABS to the caliper?
Note, that when the problem occurs on the road, I pull over and shut the engine off, and press on the brakes until the pedal starts to move, I start the car while on the pedal and it drops approximately ” - 1”. This releases the pressure on the brakes somewhat, if I continue to drive the car after a few minutes or so they break free and I am able to continue with no more problems, which is why I have not made it to my driveway with the problem in tack.
Any other test I can do to help isolate this problem? Further, has the ABS or master cylinder ever been the cause for this type of problem? Sincerely, Paul
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Monday, April 5th, 2021 AT 12:21 PM
Tiny
CARADIODOC
  • MECHANIC
  • 33,699 POSTS
The cause we dread finding is a blocked master cylinder. The clue here is when you loosen the steel lines at it, you'll get a little spurt of fluid, then the brakes will release. If this happens right after another repair in that area, typically the power booster or the brake light switch was replaced, we'd expect to see something holding the brake pedal down too far. The switch could be over-adjusted or the push rod coming out of the booster is adjusted too long. Most push rods on domestic cars aren't adjustable. This is more of an import thing.

When nothing was done that affects the brake pedal, the next thing we look for is brake fluid contaminated with a petroleum product. The rubber compound used with brake fluid is destroyed by engine oil, power steering fluid, transmission fluid, and axle grease. Professionals even wash their hands with soap and water before handling rubber brake parts to avoid getting fingerprint grease on them.

When a petroleum product enters the brake fluid, it causes rubber parts to swell and become mushy. The first visual clue is the rubber bladder seal under the reservoir cap is blown up and mushy, and can't be poked back into the cap. The same thing happens to the rubber lip seals in the master cylinder. They grow past the fluid return ports and keep the brake fluid trapped. As the fluid warms up and expands, since it is trapped, it applies the brakes harder and harder. That trapped fluid can be released by opening the hydraulic system anywhere, including the lines right up by the master cylinder.

The only proper repair for contaminated brake fluid is to remove every part that has a rubber part that contacts the fluid, flush and dry the steel lines, then install all new parts. If any part is not replaced, the contamination will leach out of the rubber part and recontaminate the new fluid and parts. This includes calipers, wheel cylinders, rubber flex hoses, master cylinder, reservoir cap, and combination valve. When the car has anti-lock brakes, the repair includes the hydraulic controller. Trucks and minivans usually use a rear height-sensing proportioning valve. Those have rubber seals and o-rings, so it must also be replaced. The cost of this repair often is more than the value of the vehicle.

The good news is when the brakes do not release when loosening the lines at the master cylinder. That means the fluid is being trapped somewhere else, and the cause is most likely not due to fluid contamination. This could be caused by the ABS hydraulic controller, but we don't hear about that very often. You'd find this the same way by loosening the steel lines leaving it that go to the wheels.
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Monday, April 5th, 2021 AT 12:21 PM

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