# What causes a car brake pedal to "sink"? **URL:** \ **Category:** Factual Questions\ **Created:** [July 19, 2013, 2:50pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937 "2013-07-19T14:50:53Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [July 19, 2013, 2:50pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/1 "2013-07-19T14:50:53Z")
Sometimes, while I’m stepping on the foot brake, I feel it “soften” and sink towards the floor. When learning to drive I thought it was the instructor stepping on his brake (the front passenger seat in learner cars have a foot brake too), but I felt this in normal cars as well. What’s going on? Additional: does the foot brake work if you turn off the engine? I found out they become MUCH harder to step on, but if I just forced it down, would it stop the car from moving? I thought it became harder because brakes are also powered, like steering. ---
**Author:** ![SanDiegoTim](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@SanDiegoTim](https://boards.straightdope.com/u/SanDiegoTim)\ **Post date:** [July 19, 2013, 2:57pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/2 "2013-07-19T14:57:11Z")
W/regard to the “sinking” you describe, I’d look first at worn pads/shoes and then to a bad master cylinder. Could be low fluid in master cylinder or a leak in the system, too. As to why the pedal is harder with the engine off, it’s because power brakes are vacuum assisted. When the engine is off there is no vacuum. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [July 19, 2013, 2:58pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/3 "2013-07-19T14:58:32Z")
This can happen if the master brake cylinder is starting to fail. ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [July 19, 2013, 2:59pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/4 "2013-07-19T14:59:55Z")
Worst case, a leak in the hydraulics, either the brake lines or cylinders. This is typically a short term condition characterized by a lack of stopping power, puddles of brake fluid under the car and trees embedded in your grille. Most common cause, air in the hydraulic lines. If the pedal is soft and “bouncy” or “spongy” overall, the system likely needs to be bled of air. It’s a pro’s job to do it right unless you have the tools and experience. (Yes, it’s one of the simpler jobs in most How To books, but it’s easy for an amateur to screw up and cheap to have done.) If it’s a firm(ish) pedal overall that slowly sinks under continued pressure, it’s likely a leaky seal in the master cylinder. Have a pro evaluate it and look at a rebuilt or replaced master cylinder (followed by a full-system bleed). ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [July 19, 2013, 3:03pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/5 "2013-07-19T15:03:56Z")
> [@SanDiegoTim](#): > > As to why the pedal is harder with the engine off, it’s because power brakes are vacuum assisted. When the engine is off there is no vacuum. Cars are typically equpped with a vacuum reservoir to provide the vacuum needed by various devices (such as the brakes) during periods when the engine isn’t producing much throttle (e.g. during wide-open-throttle acceleration). If you look under the hood you can usually find a cylinderical or spherical vessel about the size of a grapefruit; that’s the reservoir. When you shut the engine off, the reservoir typically holds enough vacuum to provide an assist for one or two firm presses of the brake pedal; after that, the vacuum is pretty much gone, and you get no assist. The footbrake still works under these conditions, but because there’s no vacuum-assist, all the braking effort has to come from you. ---
**Author:** ![somepumpkin](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)\ **Post date:** [July 19, 2013, 3:04pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/6 "2013-07-19T15:04:18Z")
Tim is right about the power brakes, and he’s also wise to advise you to check the fluid in the master cylinder. The main reason that the pedal would continue to sink is if there is air in the system. Air is compressible, but fluid is not. So, pressing on a bubble of air would feel “squishy” (like bubble wrap), but pressing on the fluid would feel solid (like a full can of beer). Additionally, you may be leaking hydraulic fluid somewhere. Be sure to check (or have your mechanic check) for this. (oh, i see the Elf and Barbarian answered before i hit “submit”. I completely agree w/ them, too.) ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 19, 2013, 3:13pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/7 "2013-07-19T15:13:07Z")
Take your car in for service before you kill someone. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [July 19, 2013, 3:28pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/8 "2013-07-19T15:28:17Z")
Oh, is it that serious? I don’t have a car, but I have the impression it was quite common. Hmm maybe I’m remembering wrongly. It didn’t sink immediately, but after holding it for a while, like described in [Master Cylinder – Auto Repair Help](http://www.auto-repair-help.com/automotive_maintenance/master_cylinder.php) “the brake pedal slowly sinking to the floor during periods of light brake applications, such as sitting at a stop”. I think the car that would creep with the handbrake applied is more disturbing… ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [July 19, 2013, 3:39pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/9 "2013-07-19T15:39:07Z")
What you describe is a leaky master cylinder. It can be overcome, in the short term, by releasing and re-applying the brakes, but that leaves a considerable safety hazard in many traffic situations. NOTHING about brakes is trivial. They work as well as they can - as well as they were designed to - or don’t drive the vehicle. And while the repairs are often simple on one level, subtle mistakes and problems can cause immediate failures. I have worked on a wide variety of brake systems and the one thing about them all is that they are not predictable - a simple job can turn into a full rebuild, and what seems major can sometimes be fixed in minutes. But you HAVE to know what you’re doing and HAVE to have the experience to make some careful judgments. Unlike the idiot who rearended me at 35+ MPH on his first test drive after replacing the master cylinder. On a fast and busy six-lane road. ---
**Author:** ![Gbro](https://avatars.discourse-cdn.com/v4/letter/g/c6cbf5/32.png) [@Gbro](https://boards.straightdope.com/u/Gbro)\ **Post date:** [July 19, 2013, 3:52pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/10 "2013-07-19T15:52:28Z")
> [@AaronX](#): > > Oh, is it that serious? I don’t have a car, but I have the impression it was quite common. Hmm maybe I’m remembering wrongly. It didn’t sink immediately, but after holding it for a while, like described in [Master Cylinder – Auto Repair Help](http://www.auto-repair-help.com/automotive_maintenance/master_cylinder.php) “the brake pedal slowly sinking to the floor during periods of light brake applications, such as sitting at a stop”. > > I think the car that would creep with the handbrake applied is more disturbing… While disturbing and should not happen, the hand brake is much different in that it mechanically operates the rear brakes which are secondary braking and the mechanical linkage when not in proper adjustment will not nessesarily mean the brake will not operate hydraulically. So keep the wheels turned to the curb. 😉 Someone posted to pump the brakes if they feel flat or spongy, well that’s good advice, however if a trooper sees the brake lights flashing you might get a visit. ---
**Author:** ![Morgenstern](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@Morgenstern](https://boards.straightdope.com/u/Morgenstern)\ **Post date:** [July 19, 2013, 3:53pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/11 "2013-07-19T15:53:34Z")
> [@TriPolar](#): > > Take your car in for service before you kill someone. This. Most likely your brakes are worn. Worn brakes can cause major problems when it comes to stopping your vehicle. Have them inspected before they fail. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [July 19, 2013, 3:56pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/12 "2013-07-19T15:56:04Z")
> [@AaronX](#): > > Oh, is it that serious? If it’s just air in the lines from not being properly bled, it’s not that serious. If it’s anything else though, it means that something is starting to fail and the next step is a sudden and complete loss of brakes while you are trying to stop, which usually results in you slamming into whatever you were slowing down to avoid. If you are quick and think well in sudden emergencies you might have the presence of mind to pull the emergency brake before you actually hit something, otherwise, WHAMMO. So yes, it’s that serious. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [July 19, 2013, 4:00pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/13 "2013-07-19T16:00:45Z")
> [@TriPolar](#): > > Take your car in for service before you kill someone. Mainly this. If you have to ask, you’re not the sort who works on brakes. 🙂 And unless you’ve done work on the system recently AND you don’t have a puddle of brake fluid under the car, it’s not an air bubble, it means the master cylinder is hosed. I replace those for a 12-pack and parts. For another 12-pack I’ll freshen up your callipers. Air bubbles don’t come from nowhere–they come from holes in the line or improper bleeding after service. And brake fluid doesn’t evaporate. If your fluid is low you’ve got a problem. Look for stains around the master cylinder, usually at the firewall. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [July 19, 2013, 4:41pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/14 "2013-07-19T16:41:17Z")
Pro tip: Give him the 12 pack after he finishes, not before. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [July 19, 2013, 4:49pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/15 "2013-07-19T16:49:25Z")
_sigh_ yeah…probably not a bad idea. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [July 19, 2013, 4:58pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/16 "2013-07-19T16:58:17Z")
We had several 1 ton fords in our fleet. No complaints about breaks but I noticed when testing the pedal after a break job the pedal would sink. I tried several other vehicles and they would all sink. Using the force on the pedal that we normaly use at a stop light they would not sink and the vehicles had no stopping complaints. Even the newer vehicles did it. Don’t ask me where the fluid was bi-passing because I have no idea but they never gave us any trouble. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [July 19, 2013, 5:14pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/17 "2013-07-19T17:14:22Z")
> [@AaronX](#): > > Oh, is it that serious? If you don’t like hitting trees or the rear ends of other cars, yes it’s quite serious. > [@](#): > > I don’t have a car, but I have the impression it was quite common. No, the symptom described below is not common in the sense of being normal for a number of vehicles. It indicates a problem. > [@](#): > > It didn’t sink immediately, but after holding it for a while, like described in [Master Cylinder – Auto Repair Help](http://www.auto-repair-help.com/automotive_maintenance/master_cylinder.php) “the brake pedal slowly sinking to the floor during periods of light brake applications, such as sitting at a stop”. This is the classic symptom of a faulty brake master cylinder leaking internally. If there’s an external leak or air in the system, the pedal will sink immediately and consistently (every stop). Certain components being worn can cause increased pedal travel before it feels firm, but worn components will not cause the pedal to sink after application. ---
**Author:** ![SanDiegoTim](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@SanDiegoTim](https://boards.straightdope.com/u/SanDiegoTim)\ **Post date:** [July 19, 2013, 8:54pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/18 "2013-07-19T20:54:39Z")
“Air is compressible, but fluid is not.” - Excellent explanation. ---
**Author:** ![Chessic\_Sense](https://avatars.discourse-cdn.com/v4/letter/c/7c8e57/32.png) [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)\ **Post date:** [July 19, 2013, 9:02pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/19 "2013-07-19T21:02:26Z")
> [@somepumpkin](#): > > Air is compressible, but fluid is not. > [@SanDiegoTim](#): > > “Air is compressible, but fluid is not.” - Excellent explanation. Nitpick: Air, like all gasses, _is_ a fluid- anything that flows is a fluid. You meant to say ‘liquid.’ ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 19, 2013, 9:16pm UTC](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937/20 "2013-07-19T21:16:52Z")
> [@Chessic\_Sense](#): > > Nitpick: Air, like all gasses, _is_ a fluid- anything that flows is a fluid. You meant to say ‘liquid.’ In context he may have been referring to brake fluid. [Next page](https://boards.straightdope.com/t/what-causes-a-car-brake-pedal-to-sink/663937.md?page=2)