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Car brakes "slip" while idling?
May 27, 2012 5:15 AM   Subscribe

My car's brake pedal slowly depresses almost all the way to the floor while my foot is resting on it at a stoplight. It never used to do this. What's going on?

If I'm not careful to hold pressure on the pedal as it moves toward the floor, the car will start to roll if I'm idling on an incline. This is a 1994 Acura Integra, if it's relevant.
posted by killdevil to Travel & Transportation (30 answers total)
 
This happened to me and it turned out I needed to have the master cylinder replaced. Take it to a mechanic to confirm.
posted by matrushka at 5:18 AM on May 27, 2012


Seconding master cylinder. Happened to me in a 1990 Ford Tempo.
posted by skittlekicks at 5:30 AM on May 27, 2012


3rding master cyl/hydraulics. I've had cars get to where you could put the pedal to the floor because the pads were about worn (this leads to the hydraulic pressure being down a touch because of the travel of the pads) but if the thing is letting the car slip, you are not far from a total brake failure. Get thee to a shop now!
posted by randomkeystrike at 5:54 AM on May 27, 2012 [1 favorite]


Had this happen once, take it to a mechanic right now before your brakes fail. Any brake problem is dangerous to let go.
posted by mermayd at 5:57 AM on May 27, 2012 [2 favorites]


When this happened to me, turns out I was leaking brake fluid rapidly. Either way, to the shop immediately! Don't fuck around with brakes.
posted by Wuggie Norple at 5:59 AM on May 27, 2012 [1 favorite]


Your car uses pressurized fluid (hydraulics) to clamp brake pads against the wheel rotors. The tube or cylinder that holds the fluid between your brake pedal and the brake pads has likely developed some defect that allows the pressure to dissipate. It could be a leaky hydraulic tube, or as mentioned above (and more likely based on the symptoms you describe) the master cylinder.
Here's a diagram of an hydraulic braking system.
posted by carsonb at 6:01 AM on May 27, 2012


Nthing master cylinder. Take your car to a mechanic ASAP. It shouldn't be an expensive fix.
posted by Giggilituffin at 6:18 AM on May 27, 2012


It's really fun when this happens as you're driving. Go directly to the mechanic.

(If it does happen while you're driving, pump your brake or (carefully) pull the e-brake.)
posted by punchtothehead at 6:34 AM on May 27, 2012


Please take your car DIRECTLY TO YOUR MECHANIC --- loss of braking ability isn't anything to let slide. Even if this ended up expensive (which it probably won't!), it'd still be cheaper to get it fixed now than risk the inability to stop your car in a (pardon the pun) crunch.
posted by easily confused at 6:41 AM on May 27, 2012


Nthing master cylinder. Nthing it being scary as hell when it fails while you're driving. Note also that, pending an actual diagnosis and depending on the configuration of your engine, this is a repair you can easily do yourself.
posted by Holy Zarquon's Singing Fish at 6:43 AM on May 27, 2012


Please take your car DIRECTLY TO YOUR MECHANIC

Locate the parking brake and remind yourself to pull it if the brakes fail in traffic.
posted by StickyCarpet at 6:44 AM on May 27, 2012 [1 favorite]


A coworker of mine once put his van through a convenience store window when his brakes failed. Get it fixed now. (And yes, it's probably the master cylinder).
posted by jon1270 at 7:04 AM on May 27, 2012


this is a repair you can easily do yourself.
posted by Holy Zarquon's Singing Fish

No. Unless you are an experienced home mechanic this is NOT something you SHOULD do yourself. You can, but unless you are very careful in following the exact procedures required you can create serious dangers. Brakes are not an area for home learning experiences.
posted by uncaken at 7:09 AM on May 27, 2012 [2 favorites]


I had this happen recently, it was a rear wheel cylinder rather than the master cylinder. Regardless, get to a mechanic ASAP. This is nothing you should be ignoring or postponing.
posted by tommasz at 7:17 AM on May 27, 2012


If your car is parked, consider towing it to a mechanic.
posted by jeffamaphone at 8:22 AM on May 27, 2012 [2 favorites]


brake pedal slowly depresses almost all the way to the floor while my foot is resting on it at a stoplight

...sometimes accompanied by (depending on your vehicle) a quantity of brake fluid leaking out, maybe onto the ground, or inside, around the brake pedal. At this point only your emergeny (or hand- ) brake will work, because it uses a mechanical linkage. As others have indicated, next stop should be your mechanic's shop.
posted by Rash at 8:38 AM on May 27, 2012


Happened to me in college - my mechanic wouldn't take the car in after the estimate and had me drive it home, then back a week later. Driving back after that week was one of the scariest experiences of my life - it was clear my brakes were not properly functional. Chalked that up to a life lesson - never drive a car with brake issues! - and to never, ever going to that scumbag mechanic ever again. It was, for the record, initially a problem with my back brake cylinder and then became a problem also with the master cylinder. Apparently, by waiting, I made the problem worse to the tune of $1k or so.

In sum: it sounds like a brake cylinder, if not the master, and please have your car towed in to your mechanic. It's not worth the risk and the gray hairs you may sprout along the way.
posted by AthenaPolias at 8:51 AM on May 27, 2012


Yes, definitely master cylinder. And no, this is not a DIY repair for inexperienced home mechanics. The friction parts (pads & rotors basically) are a good easy-to-moderate DIY project, but anything involving the hydraulics (master cylinder, calipers/pistons, etc) should be left to more experienced DIYers and professionals.

I would recommend towing for any car with a hydraulic problem. It might not fail, but if it does the results could be catastrophic for not only yourself, but others on the road as well.
posted by deadmessenger at 9:10 AM on May 27, 2012 [1 favorite]


When I replaced my master cylinder, it was a small plastic vessel under the hood that unscrewed (and the replacement screwed back in) without having to disassemble anything or even touch the actual brakes. I'm beginning to think that's less than typical.
posted by Holy Zarquon's Singing Fish at 9:16 AM on May 27, 2012


The dashboard light that says BRAKE in bright red letters turns on in two situations: when your parking brake is engaged and when your hydraulic fluid levels are low. If that light is not on, it's likely you're not yet critically low on fluid. If it is on, you should fill your brake fluid reservoir before driving again, as that light being on when your parking brake is not engaged indicates imminent brake failure.
posted by u2604ab at 9:51 AM on May 27, 2012


Look for puddles under where you park your car to try and see if the fluid is leaking. Also check the brake fluid reservoir to see if the level has gone down. I can't tell you it's safe to drive the car, because it probably isn't. But if you insist on driving the car, make sure that reservoir is kept full.

The brake system goes up to very high pressures during hard stops. You might well be fine during normal operation for a little while, but if you have to jam on the brakes, you might find that the pedal suddenly drops to the floor. Which would, at the very least, ruin your day.


When I replaced my master cylinder, it was a small plastic vessel under the hood that unscrewed (and the replacement screwed back in) without having to disassemble anything or even touch the actual brakes. I'm beginning to think that's less than typical.


That was probably the master cylinder reservoir, which just holds extra fluid for the system.
posted by gjc at 9:56 AM on May 27, 2012


Locate the parking brake and remind yourself to pull it if the brakes fail in traffic.

Better yet, pull the parking brake while driving on a clear stretch of road. The considerable time it takes you to stop will convince you to see a mechanic.
posted by weapons-grade pandemonium at 10:23 AM on May 27, 2012


Thanks for the responses, everyone. I'm going to take it to the shop ASAP. One thing that's curious: the issue seems intermittent, and I notice it more when I've just started the car.
posted by killdevil at 10:32 AM on May 27, 2012


The pressure on your brake hydraulic system is already low, and when the fluid is cold (when your car has been off for a while everything cools off) it contracts, lowering the pressure even more.
posted by carsonb at 10:43 AM on May 27, 2012 [1 favorite]


See, I had this problem a few months ago, and when I took it to two different mechanics, both said it's normal and doesn't mean that anything major is bad...
posted by These Birds of a Feather at 11:23 AM on May 27, 2012


Thanks for the serendipitous question, as this just happened to me about five minutes ago when I pulled out of the garage. (I promptly pulled back in. Mechanic tomorrow!)
posted by thomas j wise at 12:39 PM on May 27, 2012


Brake master cylinder. This is a very important repair. If it really just now started doing it, you may have a week or two, but given that if it fails completely your brakes won't work at all you should not risk it. Back when I was a teen a friend of mine let his MC go because he preferred to spend money on a couple of 15" subwoofers. One morning he came to the stop sign at the end of his street and rolled out into the busy highway because his brakes quit.

It was a bad scene, to be sure.

Last I checked, the part on most Hondas was around $120. Now that I've just checked again, I see an aftermarket version for around $80. When I had this happen, I knew a mechanic at the dealer who was happy to replace it for $20. It took about 5 or 10 minutes. If the mechanic knows how to properly bench bleed the master cylinder, it does not require bleeding the entire brake system. It requires removing three or four screws, pulling the old one out, and putting the new (bench bled) one in its place and reattaching the screws. If you can carefully follow instructions, you could probably do this yourself.

BTW, in my experience fluid does not leak out of the system in this situation. In the master cylinder, there is a piston that pushes fluid through the lines to apply the brakes. On this piston, there is a rubber seal. Over time, depending on how well maintained the car was, either the rubber seal will break down or the metal in the master cylinder will get pitted. In either scenario, fluid leaks right past the rubber seal, so the brake pedal sinks slowly towards the floor as you continue applying pressure. The lack of external leakage may be a Honda/Acura specific thing. If you have a manual transmission, however, you'll find that the clutch master cylinder does indeed leak onto your floorboard when it fails.

If you live in a relatively rural area and have low speed roads and if I'm correct that your Integra has a cable-actuated parking brake, you can drive it (very, very slowly) to the mechanic. Otherwise, get it towed.
posted by wierdo at 3:47 PM on May 27, 2012 [3 favorites]


One thing that's curious: the issue seems intermittent....
This is exactly what happened to me when my master cylinder was failing. Sometimes the pedal was OK, sometimes it descended to the floorboard.
posted by exphysicist345 at 7:40 PM on May 27, 2012


If you need to drive the car, and your brakes suddenly give out, remember that the handbrake can be gently applied to bring a moving car to a stop.
posted by jannw at 7:53 AM on May 28, 2012


The pressure on your brake hydraulic system is already low, and when the fluid is cold (when your car has been off for a while everything cools off) it contracts, lowering the pressure even more.

This doesn't make any sense. Fluid in your braking system is at zero pressure at all times until you press the brakes - hot or cold. It is likely that the master cylinder warms up by virtue of it just being in the engine bay and the seal expands with the heat more than the metal cylinder and just makes a better seal. The fluid doesn't get all that warm unless under hard driving through the brakes heating, really, and even then only to any major degree at the wheel end of the line. Either way you won't really see any appreciable difference in line pressure between hot and cold.

OP: Yes, this is a mastery cylinder, it is not at all an easy job (Holy Zarquon's singing fish is incorrect - he replaced a master cylinder reservoir, not the master cylinder itself). Get this taken to a mechanic as soon as possible and describe the symptoms exactly as you have here. Don't take the car from the shop until they have fixed it just in case they can't replicate the issue. If the issue is repeatable in any conditions, then make sure you know how to 'make it do it' or give them information as you have on when it is more prevalent so that it can happen when they are looking at it.
posted by Brockles at 7:59 AM on May 28, 2012


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