Now you're starting to add some new problems to the story. The issue is whether the stuck brake will release, and it did, so the list of suspects is real short. There's no need to bleed the system. That is done to get air out of the fluid. Air compresses and will prevent the brakes from applying. That's the opposite of the problem you're having, so forget about bleeding.
The power booster is run on engine vacuum. The farther you push the pedal, the more stored vacuum in the booster will be exhausted, and the incoming air will have to be pulled out by the engine. That mimics a vacuum leak, and vacuum leaks result in increased engine speed without a corresponding increase in power. You have too much air to go with the fuel the Engine Computer is commanding. In response to the increased engine speed, the computer will try to lower it back to the desired idle speed. It lowers idle speed by reducing the amount of air bypassing the throttle blade, AND by cutting back on the amount of fuel. It is programmed to know how much fuel to provide based on the amount of air coming in, but it doesn't know about that extra air coming from the power booster. Cutting back on fuel is what is causing the low idle speed. That is a secondary symptom that's not related to the sticking brake problem.
I cringed when I read your comment about pushing the brake pedal to the floor. That must never ever be done unless the master cylinder is less than about a year old. Crud and corrosion build up in the lower halves of the two bores where the pistons don't normally travel. When you push the pedal to the floor, either during bleeding or when you're surprised by a ruptured hose, running the pistons past half way runs the lip seals over that corrosion and can rip them. At that point you'll have internal leakage where the brake fluid can sneak past the seals and it won't get pushed down to the wheels to apply the brakes. Often the symptoms don't show up for two or three days. The symptoms are the pedal will go too far down to the floor or it will slowly sink down when you're holding steady pressure on it, as in when sitting at a stop light. The only fix for that is a rebuilt master cylinder. Mechanics pretend there's a block of wood under the pedal and never go more than half way down with the pedal unless it's a new master cylinder that won't have any corrosion in it. Pedal-bleeding with a helper who pushes the pedal all the way down is a common mistake inexperienced beginning mechanics often make.
The suspects now are anything that is blocking the trapped brake fluid from getting past the point where you loosened the steel line and returning into the reservoir. There's only one thing that can be, and that's those lip seals on the two pistons. Now we have to determine why those seals aren't moving back far enough to expose the return ports. There's two possibilities. The brake fluid is contaminated with a petroleum product or something is holding the brake pedal down a little.
A single drop of power steering fluid, transmission fluid, axle grease, or engine oil is all it takes to become a disaster. Experienced brake specialists even wash their hands before handling brake parts to avoid getting fingerprint grease in the fluid. About a dozen cars were damaged at a national chain store about 20 years ago when the mechanic wiped out a funnel used previously for engine oil, and used it to fill a "bleeder ball" from a five-gallon pail of new fluid. The residue in the funnel was all it took to do all that damage.
Every year I did a demonstration for my students to show what happens when the fluid is contaminated. I had two beakers with fresh, new brake fluid, and I put a new rubber seal in each one. In one beaker I added one drop of power steering fluid, then we watched them for a week. After that week, the seal in the contaminated fluid had grown by about 25 percent and was real soft and mushy. When that happens in the master cylinder, the seals grow past the fluid return ports and blocks them. The result is self-applying brakes that lock up, then release when they cool down.
Here's the secret though. The ONLY proper fix is to drain the old fluid, remove every part that has rubber in it that contacts the fluid, flush and dry all the steel lines, then install the new parts and new fluid. If any rubber part is not replaced, the contamination will leach out of it and recontaminate the new fluid and all the new parts. That means new calipers on the front, new wheel cylinders or calipers on the rear, three or four new rubber flex hoses, master cylinder and the bladder seal under the cap, and the combination valve with its rubber o-rings. Trucks and minivans usually also have a height-sensing proportioning valve in the rear that has rubber o-rings, and if the vehicle has anti-lock brakes, the hydraulic controller is full of seals and o-rings. Those are expensive enough that the vehicle could be considered a total loss.
Lets hope there's a mechanical issue holding the brake pedal down too far. There's two possibilities again for that. Some power boosters have an adjustable push rod, and on the replacement booster it could be adjusted out too far. That could be the case with a replacement booster, but not the original one unless someone monkeyed with it. The second suspect is the brake light switch is misadjusted and the button is holding the pedal down. It only takes 1/8" to cause this problem. To identify if this is a mechanical problem, the next time the brakes lock up, loosen the nuts that hold the master cylinder to the power booster, and pull the master cylinder away from the booster by about 1/8". If the brakes release, start by looking at the brake light switch.
Saturday, August 16th, 2014 AT 9:35 PM
