Is it the master cylinder?
September 5, 2019 3:33 PM   Subscribe

While I was driving recently, my brakes went all the way to the floor before fully stopping the car. I drove home at a crawl as the problem continued, then they suddenly started working normally again. I explained this to my usually thorough and helpful mechanic, who has been driving it to test, can’t replicate or find any problem and now insists that it’s fine. Is it weird that I’m reluctant to drive this car? Would you drive it? If it is the master cylinder as the internet suggests, shouldn’t the mechanic be able to figure that out? And also if it is, if the brakes fail again will it be gradual again, or are they likely to fail completely?
posted by lgyre to Travel & Transportation (11 answers total) 2 users marked this as a favorite
 
This sounds like the time I failed to completely disengage the parking brake, and managed to get on the highway and drive for a while with it partially engaged. By the time I tried to hit the brakes, friction had created a lot of heat, and they barely worked. It was terrifying. After cooling down, the brakes worked again, but I did ultimately have to replace the rear pads.
posted by qxntpqbbbqxl at 3:47 PM on September 5, 2019 [1 favorite]


Is it weird that I’m reluctant to drive this car?

Not in the slightest. I would refuse to drive it without a satisfactory explanation for what happened and proof it is fixed. I grew up in aviation safety culture and am consistently appalled at the world of driving.
posted by automatronic at 4:19 PM on September 5, 2019 [6 favorites]


More information would be helpful....

What is the make and model and year of the car?

When the problem happened, did you try pumping the brake and did that seem to restore any pressure?

Do your brakes make any sounds?

Does your car pull in any direction when you stop?

Does the brake pedal pulsate when you stop?

When was the last time you had pads and rotors done?
posted by spitbull at 4:32 PM on September 5, 2019 [1 favorite]


I ask because the brake pedal basically cannot go down to the floor if there isn't either a fluid leak or air in the lines. If they return to normal, they should have enough fluid. If they have air in the lines, pumping them should fix that.

Step one to eliminate these causes would be to bleed and fill the brake lines, make sure the fluid level is right (overfilling is bad), and checking to see if any fluid leaks are making spots or puddles under the car. A fluid leak could indeed be a sign of a failing master cylinder.

I'm assuming no dashboard warning lights have illuminated. Did your guy pull any codes?

I'd get another mechanic to have a look. Intermittent problems can be hard to diagnose, but you can't mess around with brake failure. Fresh eyes on the problem can help.
posted by spitbull at 4:37 PM on September 5, 2019 [1 favorite]


No such personal experience, but my understanding is that extreme high temperatures at the brakes or brake lines, e.g. due to driving for a long time with the parking brake engaged, can boil the brake fluid and create compressible bubbles that make the brakes behave this way.
posted by jon1270 at 5:03 PM on September 5, 2019 [1 favorite]


>More information would be helpful....

>What is the make and model and year of the car?

>When the problem happened, did you try pumping the brake and did that seem to restore any >pressure?

>Do your brakes make any sounds?

>Does your car pull in any direction when you stop?

>Does the brake pedal pulsate when you stop?

>When was the last time you had pads and rotors done?

Mostly a lot of “I don’t know.” It’s a Honda Civic, early 2000s. Didn’t notice any sounds, pulling or pulsing. Also not sure about the pads and rotors, but the car did get a tuneup a couple of weeks before this. Unfortunately I didn’t think to pump the breaks, but I did press the pedal repeatedly over the course of testing and slowly driving home, which didn’t seem to change anything until it suddenly did. Mechanic says he checked the master cylinder and fluid, fwiw.
posted by lgyre at 5:31 PM on September 5, 2019


Don't just look at the brake fluid, chuck it out. Flush the system THOROUGHLY to purge as much of the old stuff (I won't call it brake fluid because it is highly that it is not fit for that purpose any more), and replace it with premium grade fluid of the correct spec. Old deteriorated brake fluid is the most likely culprit here. While you are in there check the pads too - they don't deteriorate but do wear ;-) .

qxntpqbbbqxl, that does not compute - the dragging handbrake should have been easily overcome by the front brakes, which normally do the hardest work and are therefore more powerful than the rears. Unless you too had old fluid, and it was boiling in the rear callipers? And the fronts started boiling in sympathy, as they were then being asked to do a lot more work?

Everybody should be aware that brake fluid absorbs moisture from the atmosphere, and that moisture boils at a much lower temp than purpose-made brake fluid. Flushing out the old stuff and replacing with new (NOT just topping it up) should be be regular part of your car maintenance - check your car manual for times/distances.
posted by GeeEmm at 7:18 PM on September 5, 2019 [2 favorites]


From the way you're describing it, it sounds more like a linkage problem between the pedal and the brake hydraulic system. It looks like Honda used a few different designs for the system in the early 2000s, but just in general I would examine the mechanical link between the break pedal and the master cylinder.

I would drive the car but I would be ready with the hand brake in case the problem happened again.
posted by Tell Me No Lies at 8:30 PM on September 5, 2019


It is not weird that you are reluctant to drive the car when the problem you have experienced has not been definitively explained and fixed. Personally, I would not drive it other than what was necessary to find the problem. I do think it sounds more like a linkage issue, as Tell Me No Lies says, and I would look very carefully at that part of it, but I would also have the brake fluid changed and brake system bled unless I knew it had been done in the last 12 months.
posted by coppertop at 5:17 AM on September 6, 2019 [1 favorite]


Unrelated to your brake issues, but if you have an early 2000s Honda, you should also definitely find out of there is an open recall on it for exploding Takata airbags.
posted by spitbull at 1:27 AM on September 7, 2019


I wouldn’t drive it until I had an explanation. I would look at this as the universe giving you a ... chance ... to avoid death or serious injury. If your mechanic can’t come up with a decent theory, find one who can. The problem with this is that you run the risk of trading an honest but slightly-incompetent mechanic for an incompetent bullshitter.

The complete flush-and-fill followed by inspection for leaks would be cheap and a great idea, by the way.
posted by Gilgamesh's Chauffeur at 5:56 AM on September 7, 2019 [1 favorite]


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