Very hard brake pedal with locking brakes

Tiny
FREEMYBRAKES
  • MEMBER
  • 2002 CHRYSLER SEBRING
  • 2.7L
  • V6
  • 2WD
  • AUTOMATIC
  • 117,000 MILES
I recently replaced master cylinder with new one. Initially was perfect, but after driving for a day; brake pedal gets to where there is little to no movement and brakes get so tight I cannot roll even with power to drive. I crack the line at master cylinder and there is extreme pressure. After relieving the pressure, it frees the calipers and I can once again roll. Sometimes for just a mile and others for 30 miles, but they will lock again.
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Saturday, January 4th, 2020 AT 8:25 AM

6 Replies

Tiny
CARADIODOC
  • EXPERT
How long ago was the master cylinder replaced, and what was the reason? Next time this happens, loosen the mounting nuts and let the master cylinder move forward from the power booster 1/8th inch. If that lets the brakes release, the brake light switch is holding the brake pedal down a little, or the booster's push rod is adjusted too long. Adjustable push rods are typically found on import vehicles, not on domestics, but check that any way.

Loosening the steel lines is the right test to verify pressurized brake fluid is being trapped and is not able to release back into the reservoir. The only thing that can cause that is the rubber lip seals on the pistons have moved down into the bores of the master cylinder and are blocking the fluid return ports. That is what is supposed to happen when we want to push that fluid to the brakes with the pedal, but what is not happening is the seals aren't moving back far enough when the pedal is released to let that fluid flow back from the wheels.

Besides the push rod issue, this is often caused by brake fluid contaminated with a petroleum product. That will cause rubber parts to swell and become mushy. If that is what happened to the original master cylinder, the only proper repair for that is to remove every part that contains a rubber part that contacts the brake fluid, flush and dry all the steel lines, then install new parts. If only the master cylinder was replaced, it typically takes about a week for the contamination to leach out of the other parts and recontaminate the new fluid. That includes all of the calipers, wheel cylinders, rubber flex hoses, combination valve assembly, height-sensing proportioning valve, when used, (those are mainly used on minivans and light trucks), and the master cylinder. If the vehicle has anti-lock brakes, the hydraulic controller must also be replaced because it has rubber O-rings and seals inside it.

The most common contaminants include power steering fluid, transmission fluid, engine oil, and penetrating oil. Years ago when it was common to repack front wheel bearings, the grease residue on our fingers often found its way into the brake fluid when we went to refill the reservoir, and we popped the rubber bladder seal back into the reservoir's cap. Most brake system specialists even wash their hands with soap and water when handling rubber brake parts to avoid getting fingerprint grease on them.

This same symptom often occurs when pressurized brake fluid is trapped by a rubber flex hose, especially those that have a metal bracket crimped around them in the middle. But while the symptom is the same, you would have not found trapped brake fluid by loosening the lines at the master cylinder, as you did. With this cause, the brakes would only release when the bleeder screw on the caliper is opened.
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Saturday, January 4th, 2020 AT 10:12 AM
Tiny
FREEMYBRAKES
  • MEMBER
The reason the master cylinder was replaced is because the pedal was mushy and would fade. I took it to Firestone and they're the ones that told me that the master cylinder was bad. I changed it myself just 3 days ago and that's when this problem started. The pedal no longer fades! I'm reasonably certain that there has been no cross-contamination with petroleum products. If there is that sounds like it would probably total out this vehicle even though it's got new engine and entirely new suspension struts CV joint bushings ball joints you name it they're new.
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Saturday, January 4th, 2020 AT 10:00 PM
Tiny
CARADIODOC
  • EXPERT
Dandy. Did you check the brake light switch and the booster's push rod? It sounds like you had the typical, common master cylinder failure which can be expected on a car this age. (I just replaced mine for the same problem on my '94 Grand Voyager, two months ago).

Since the only change to your car was the master cylinder, we know it's a pretty good bet something related to it is causing this problem.
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Sunday, January 5th, 2020 AT 12:09 PM
Tiny
FREEMYBRAKES
  • MEMBER
When you say brake light switch, I dont understand how that might affect the actual brake performance. Please explain if you have time.
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Monday, January 6th, 2020 AT 4:44 PM
Tiny
FREEMYBRAKES
  • MEMBER
Also sir, I have not had the time to do additional diagnostics yet. I drive the vehicle and brakes tightened, I cracked brake line at master cylinder and it dripped the pedal nearly to the floor when braking. Then, it suddenly pumped up to very near normal after approx 25 to 30 braking and remains nearly acceptable. Is this even possible?
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Monday, January 6th, 2020 AT 4:48 PM
Tiny
CARADIODOC
  • EXPERT
When you release the brake pedal, there is a tab on the arm that pushes on the brake light switch to turn it off, similar to how closing a door turns off the dome light. Brake light switches usually have a means of adjusting them so they turn on and off at the right time. If that switch is adjusted too close to the firewall, it will hold the brake pedal down, just as if you were driving with your foot on the brake pedal.

Under normal operation, when you push the brake pedal, the piston in the master cylinder moves forward, and the first thing that happens is the rubber lip seal on it passes the fluid port going to the reservoir. At that point the brake fluid is trapped by that seal, then it has no choice but to be pushed down to the wheels as you push the pedal further. (That seal is leaking when you have "internal leakage". That results in the brake pedal slowly sinking to the floor, and you are not losing brake fluid).

When you release the brake pedal, the piston in the master cylinder comes back far enough to expose the port to the reservoir. That lets all the pressurized brake fluid return from the brakes, so the brakes release. If you instead continue to hold the brake pedal down as little as half an inch, that port remains blocked, the brake fluid remains trapped and under some pressure, and the brakes do not fully release. Instead of your foot causing that, the brake light switch can do it too. Once you continue driving with the brakes stuck partially-applied, the front disc brake pads become hot from the constant friction. That heat migrates through the caliper and into the brake fluid. That heat makes the brake fluid expand, and since it is trapped and unable to flow back into the reservoir, it builds even more pressure in the system. That higher pressure applies the brakes even harder, then they get even hotter. It's a vicious circle until the car won't even move or the brakes are smoking. This also often causes a thumping feeling in the steering wheel and / or the seat of the car. It feels like there's big blob of tar stuck to a tire.

To be fair, the brake light switch adjustment is not a likely cause of this because you weren't working in that area, so nothing related to it should have changed, but we don't want to overlook the possibility something happened to it. You may be able to identify this by stopping on a slight incline, shifting to neutral, then pulling the brake pedal up with your foot. If the brakes release, it is because the force you're putting on the brake pedal is bending the mounting bracket for the brake light switch. That proves the switch is holding the pedal down too far.

The other possibility is the power booster's push rod I mentioned, but those usually aren't adjustable on Chrysler products. That means they can't be misadjusted.

There are other places where brake fluid can become trapped under pressure, but we're beyond that by your observing the brakes release when you loosen the lines at the master cylinder. That proves for certain that is where the fluid is being trapped.

I mentioned brake fluid contaminated with a petroleum product too. The rubber parts including the lip seals are not compatible with petroleum products. It causes the rubber to swell and become soft and mushy. When they swell up, they grow past the fluid return port to the reservoir, so the brake fluid remains trapped in exactly the same way as holding your foot on the brake pedal does. That type of damage takes about a week to show up once the contamination hits the rubber seals.

I'm not sure what you meant in your last reply, but I have an idea of what might have happened. Explain that a little differently or with more detail.
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Tuesday, January 7th, 2020 AT 11:52 AM

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