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Chevy Cavalier - Bad Master Cylinder?

A friend came to town with a circa 2000 Chevy Cavalier, 135k miles. Before she embarked on the long road trip that brought her here, her husband checked the fluids and put on new brakes for her. This car has disc brakes and the front and drums in the back. At first, it was fine. But after some time driving on the highway, the braking became excessively spongy, to the point where it did not feel safe. She had to push the brake into the carpet to get the car to stop.

I drove the car around for a couple miles, and could not reproduce the problem, even with heavy brake use. The breaks were good and solid all the while I drove. I could produce another problem, that has also occurred since the new brake job; the rear passenger side drum break squeaks once warmed up and braking pressure is applied. I’m doubtful that that is related to the brake sponginess, but thought I’d mention it.

I checked the brake fluid; it’s full but green, indicating that it hasn’t been changed recently. But it’s a light color green, not dingy, and there’s no off smell.

She has to leave in the morning, back out on the road, and I live in a small town with no Saturday mechanic. I suspect a failing master cylinder, but I’m not confident that’s the problem. I’m advising she should stop in the next town with a mechanic, which is going to be about a 25 minute drive.

edit: I should clarify that I’m asking: does it sound likely that this is indeed a bad master cylinder? And a 25 minute drive should be reasonably fine? (in other words, there’s nothing here to indicate imminent brake failure?)

Why is the brake fluid green? Any chance someone diluted the fluid with anti-freeze?
That has been known to happen and brake fluid has a distinctive smell to it.

Brake pedal sponginess can certainly be caused by a weak master cylinder (anti-freeze can kill it) and on cars equipped with drum rear brakes sponginess can also be caused if the rear shoes are excessively worn and/or out of adjustment.

I hope no one added anti-freeze to it! I googled green brake fluid, and it seems a common enough issue in certain vehicles. There’s a lot of guesses as to why (such as copper corrosion), but I haven’t seen a definitive answer.

Really my question is this: to my knowledge, either a master cylinder is working or it isn’t. Can it be the case that the master cylinder works fine when it’s cold and begins to fail when it’s hot?

Yes. A master cylinder can also fail to some degree intermittently by being fine one minute and gone stupid the next.

I would advise that until this is sorted out that your friend should drive carefully and try to think ahead while in traffic or approaching traffic lights, etc.
In the event that a situation develops which requires a panic stop drop the transmission into LOW, apply the park brake, and in a worst case scenario; slam the shifter into PARK.

Slamming it into PARK raises the possibility of causing damage to the parking pawl or even worse but it’s better than ramming something at speed or sailing through a light into heavy traffic.
There’s also a good chance that doing this will hurt nothing in the transmission at all. I’ve seen this done deliberately several times and other than smoke rolling off of locked tires, no harm and no foul.

I might just reiterate the part about out of adjustment rear drum brakes being the cause of sponginess. This is normally not an issue with rear disc brakes but with drum brakes it can certainly mimic a bad master cylinder.

Very old brake fluid sometimes can appear very dark

I suppose somebody might think it’s a very dark green

I’ve never seen green brake fluid. I strongly, strongly, strongly recommend that she rent a car until she can get this TOWED to her mechanic. If she doesn’t, and my hunch about the wrong fluid being in there is correct, she stands a very good chance of finding herself suddenly without brakes unexpectedly. IMHO she should consider this vehicle very unsafe until the issue gets resolved. A system flush… a GOOD system flush… and refill with the proper fluid might be all she needs, but if she suddenly loses her brakes she may need a whole new car… and maybe even a wheelchair.

Coolant is an anticorrosive, lubricating (for the pump), fluid designed to transfer heat well. It is NOT a hydraulic fluid. Brake fluid is a hydraulic fluid. It is designed to withstand compression and transfer force. They are NOT interchangeable.

I could be wrong. But I could be right. The risk if I’m right is unacceptably high.

Spongy brakes after highway driving, then good brakes after cooling down tells me the brake fluid is being heated up and boiling at highway speeds. Maybe the fluid is contaminated and needs to be replaced, and that’s all it is. But, I think something is wrong with the brakes and making the brake fluid overheat.