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#1
Old 07-19-2013, 10:50 AM
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What causes a car brake pedal to "sink"?

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.
#2
Old 07-19-2013, 10:57 AM
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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.
#3
Old 07-19-2013, 10:58 AM
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This can happen if the master brake cylinder is starting to fail.
#4
Old 07-19-2013, 10:59 AM
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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).
#5
Old 07-19-2013, 11:03 AM
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Quote:
Originally Posted by SanDiegoTim View Post
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.
#6
Old 07-19-2013, 11:04 AM
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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.)

Last edited by shunpiker; 07-19-2013 at 11:06 AM.
#7
Old 07-19-2013, 11:13 AM
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Take your car in for service before you kill someone.
#8
Old 07-19-2013, 11:28 AM
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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 http://auto-repair-help.com/auto...r_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...
#9
Old 07-19-2013, 11:39 AM
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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.

Last edited by Amateur Barbarian; 07-19-2013 at 11:39 AM.
#10
Old 07-19-2013, 11:52 AM
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Quote:
Originally Posted by AaronX View Post
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 http://auto-repair-help.com/auto...r_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.

Last edited by Gbro; 07-19-2013 at 11:54 AM.
#11
Old 07-19-2013, 11:53 AM
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Quote:
Originally Posted by TriPolar View Post
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.
#12
Old 07-19-2013, 11:56 AM
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Quote:
Originally Posted by AaronX View Post
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.
#13
Old 07-19-2013, 12:00 PM
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Quote:
Originally Posted by TriPolar View Post
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.

Last edited by The Great Sun Jester; 07-19-2013 at 12:02 PM.
#14
Old 07-19-2013, 12:41 PM
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Pro tip: Give him the 12 pack after he finishes, not before.
#15
Old 07-19-2013, 12:49 PM
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*sigh*

yeah...probably not a bad idea.
#16
Old 07-19-2013, 12:58 PM
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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.
#17
Old 07-19-2013, 01:14 PM
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Quote:
Originally Posted by AaronX View Post
Oh, is it that serious?
If you don't like hitting trees or the rear ends of other cars, yes it's quite serious.

Quote:
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.

Quote:
It didn't sink immediately, but after holding it for a while, like described in http://auto-repair-help.com/auto...r_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.
#18
Old 07-19-2013, 04:54 PM
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"Air is compressible, but fluid is not." - Excellent explanation.
#19
Old 07-19-2013, 05:02 PM
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Quote:
Originally Posted by shunpiker View Post
Air is compressible, but fluid is not.
Quote:
Originally Posted by SanDiegoTim View Post
"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.'
#20
Old 07-19-2013, 05:16 PM
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Quote:
Originally Posted by Chessic Sense View Post
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.
#21
Old 07-19-2013, 10:25 PM
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Wow, thanks everyone, I'll take note of when it happens.
#22
Old 07-20-2013, 07:28 AM
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Quote:
Originally Posted by Chessic Sense View Post
Nitpick: Air, like all gasses, is a fluid- anything that flows is a fluid. You meant to say 'liquid.'
I realize this is GQ, but there are about a dozen ways you could have phrased this to avoid coming off so pedantic and condescending.
#23
Old 07-20-2013, 11:21 AM
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after brakes are worked on they should be tested, both driving and parking brakes, before taking the car out on the road,

if you get the brakes serviced by someone then to test find out where their car is parked. with your car 30 yards away get to a street driving speed and apply brakes with distance enough to avoid a collision.
#24
Old 07-21-2013, 05:31 AM
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You can also get spongy brake pedals (pedal fade) due to overheated brakes boiling the brake fluid. If the sponginess occurs after a period of sustained or hard braking, and passes after a short period of time, this may be the cause. You need to get the brake fluid checked/replaced as it may have contaminants that boil sooner than expected (water, generally), and you may need to modify your driving technique to avoid overheating the brakes.
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#25
Old 07-21-2013, 07:07 PM
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si, does that even happen (apart from spirited driving on a track)? How does water or anything else get into the fluid with there being a much greater problem?
#26
Old 07-21-2013, 08:00 PM
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Last time it happened to me it was when descending the Alps into Austria on a single carriageway road - steep grade, lots of turns - with an overheating engine so engine braking was limited.

Many brake fluids are hygroscopic, which means they absorb water from the air. This is why proper brake maintenance is so important.
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#27
Old 07-21-2013, 08:25 PM
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Quote:
Originally Posted by engineer_comp_geek View Post
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.
(bolding mine)

IANACM (I am not a 'certified' mechanic)

Kind of a :nitpick:...
On some makes/models of vehicles the 'emergency' brake is not... well, an actual 'emergency' brake, as much as it is a 'parking' brake.
As an example, on the Chevrolet 1/2 ton Silverado P/U the mechanical foot brake pedal (next to the sidewall) is designated as a 'parking' brake. That model of P/U (equipped w/4 wheel disc bakes) utilizes a small 'drum' type brake in the rear wheels, along with the normal disc brake. The smaller drum brake is not sufficient enough to stop the vehicle in an emergency situation (in my experience).
Granted, it's better than nothing, but I think that it's primarily meant to be used as a 'parking brake'.
For instance, when parking on an incline, the brake can be set to keep from relying on the transmission alone, to keep the vehicle from rolling.

Hopefully, Gary T will check back in and give his professional opinion on this.
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#28
Old 07-21-2013, 11:07 PM
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Quote:
Originally Posted by JBDivmstr View Post
On some makes/models of vehicles the 'emergency' brake is not... well, an actual 'emergency' brake, as much as it is a 'parking' brake.
I don't know if any manufacturer calls it an emergency brake nowadays, though I gather the term was more common decades ago. I think the fear is that the term "emergency brake" suggests it's a more or less an equivalent substitute for the service brake (the regular hydraulic brake system operated by the brake pedal), whereas it provides nowhere near as much stopping power. While it can be helpful -- as you say, better than nothing -- in some emergency situations, it's designed to be effective at keeping a parked car from rolling. It is not very effective at stopping a moving vehicle, especially at higher speeds.
#29
Old 07-22-2013, 03:57 AM
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Quote:
Originally Posted by si_blakely View Post
Last time it happened to me it was when descending the Alps into Austria on a single carriageway road - steep grade, lots of turns - with an overheating engine so engine braking was limited.

Many brake fluids are hygroscopic, which means they absorb water from the air. This is why proper brake maintenance is so important.
Si, I agree with you about proper brake maintenance.

I am curious about an overheating engine not being much use to slow down your rig in the alps. In the Colorado Rockies, I use engine braking with hot engines often. I have not experienced any problems from this practice. In fact it cools the engine fairly fast. It also saves your brakes from overheating.

As I am sure you are aware, there is an old truckers "rule" that one should descend a mountain in the same gear that was used to ascend the mountain. In other words, use engine braking to keep your speed down while descending. So why could you not have done that?

Respectfully yours, 48.
#30
Old 07-22-2013, 04:23 AM
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The overheating was due to a leaky gasket but I didn't actually know that at the time - if the revs went up (as they do under engine braking) so did the heat, and I didn't want to stress the engine any more than I had to, as we still had to get round Italy and home to the UK. I would have used the engine as a brake more in normal circumstances. I was just being cautious - I could get brakes replaced quickly - not a more serious engine problem.
#31
Old 07-22-2013, 04:34 AM
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Actually, it raises a point about modern car technology.

When I learned to drive in the 80's (in 70's era cars), we had to know about brake fade due to old brake fluid, wet brakes, air bubbles or overheating. We also knew about stuck chokes, water in the carb float, loose distributor caps, leaky ignition leads, dirty spark plugs and loose tappet noise - a whole plethora of minor technical/mechanical issues you might need to deal with on an occasional basis, or at least be aware of. A generation older than me knew about reversing up steep hills because reverse was a lower ratio than first, and first didn't have syncromesh so you had to stop to change into it.

Modern cars are much less prone to issues like those, and (it seems to me) many people are less prepared to deal with them. To be fair, though, I've probably forgotten plenty of that stuff myself.
#32
Old 07-22-2013, 06:47 AM
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Quote:
Originally Posted by si_blakely View Post
Actually, it raises a point about modern car technology.

When I learned to drive in the 80's (in 70's era cars), we had to know about brake fade due to old brake fluid, wet brakes, air bubbles or overheating. We also knew about stuck chokes, water in the carb float, loose distributor caps, leaky ignition leads, dirty spark plugs and loose tappet noise - a whole plethora of minor technical/mechanical issues you might need to deal with on an occasional basis, or at least be aware of. A generation older than me knew about reversing up steep hills because reverse was a lower ratio than first, and first didn't have syncromesh so you had to stop to change into it.

Modern cars are much less prone to issues like those, and (it seems to me) many people are less prepared to deal with them. To be fair, though, I've probably forgotten plenty of that stuff myself.
I concur on all of those points.
My first vehicle was a '74 Chevy with a 3 speed manual transmission with the shift lever on the steering column, no less!

(When was the last time you saw one of those?)
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#33
Old 07-22-2013, 07:12 AM
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Originally Posted by The Great Sun Jester View Post
si, does that even happen (apart from spirited driving on a track)? How does water or anything else get into the fluid with there being a much greater problem?
I've experienced brake fade on my motorcycle (a late-model BMW). Even with the brake fluid recently bled (i.e. moisture-free), the fluid started boiling while descending a twisty mountain road in SE Kentucky. The road had straightish sections where you could build up decent speed, but they were connected by sharp turns and switchbacks that required scrubbing off a lot of that speed. Repeat for a few minutes, and pretty soon my front brake lever started getting spongy. We stopped for a 10-15 minute brake, and lever feel returned to normal.

about 12 years ago I completely boiled the front brake during track riding. Pulled the lever all the way back to the bar with zero resistance, and achieved zero braking force. Ran off into the grass, somehow managed to get it stopped with just the rear brake. Decided I was done for the day at that point.
#34
Old 07-22-2013, 11:02 AM
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Quote 'elf
about 12 years ago I completely boiled the front brake during track riding. Pulled the lever all the way back to the bar with zero resistance, and achieved zero braking force. Ran off into the grass, somehow managed to get it stopped with just the rear brake. Decided I was done for the day at that point.
Wow! That must have been fun! [/sarcasm]
#35
Old 07-22-2013, 11:11 AM
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What causes a car brake pedal to "sink"?

It's not a witch.
#36
Old 07-22-2013, 11:53 AM
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Quote:
Originally Posted by Chessic Sense View Post
Nitpick: Air, like all gasses, is a fluid- anything that flows is a fluid. You meant to say 'liquid.'
Oops... that's probably a more-accurate way to phrase it. As Tripolar mentioned, I was referring to the brake "fluid", so i just kept that terminology. Thanks for keeping me on my toes!
#37
Old 07-22-2013, 12:28 PM
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Originally Posted by Rachellelogram View Post
I realize this is GQ, but there are about a dozen ways you could have phrased this to avoid coming off so pedantic and condescending.
I agree with your noting the tone in posts and being aware of its importance, but in this case I believe a nice GQ convention is being used appropriately.

By using the "nitpick" header, the poster can quickly zap in a quick fact or correction that is either nitpicky to the discussion, and is thereby excused from introduction and reference beyond the mere posting of fact, or may be important enough in other discussions but not helpful to get into in the current one.

Nitpicking typos or grammatical errors, for an extreme example, is often seen and quite rewarding, bur bears the risk of complete nudnik-ity and eventual withering opprobrium, if not outright social exclusion.

But now we are in ATMB territory.


Best,
Leo

Last edited by Leo Bloom; 07-22-2013 at 12:29 PM.
#38
Old 07-23-2013, 02:19 AM
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Quote:
Originally Posted by si_blakely View Post
Actually, it raises a point about modern car technology.

When I learned to drive in the 80's (in 70's era cars), we had to know about brake fade due to old brake fluid, wet brakes, air bubbles or overheating. We also knew about stuck chokes, water in the carb float, loose distributor caps, leaky ignition leads, dirty spark plugs and loose tappet noise - a whole plethora of minor technical/mechanical issues you might need to deal with on an occasional basis, or at least be aware of. A generation older than me knew about reversing up steep hills because reverse was a lower ratio than first, and first didn't have syncromesh so you had to stop to change into it.

Modern cars are much less prone to issues like those, and (it seems to me) many people are less prepared to deal with them. To be fair, though, I've probably forgotten plenty of that stuff myself.
And on this side of the Pond reversing up a hill was because of gravity feed fuel system, Unless the fuel tank was full one would run out of fuel going forward. Then the gravity fuel tank was put up on the firewall instead of under the seat.
#39
Old 07-23-2013, 01:19 PM
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Originally Posted by AaronX View Post
I think the car that would creep with the handbrake applied is more disturbing...
What do you mean creep?
Do you mean that when the handbrake applied, the car rolls when parked or creeps when it is running and in gear?
The latter tends to cause the former.
#40
Old 07-23-2013, 08:37 PM
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(Automatic) when in drive, the car would still move forward even with the handbrake applied.
#41
Old 07-24-2013, 07:40 AM
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Quote:
Originally Posted by AaronX View Post
(Automatic) when in drive, the car would still move forward even with the handbrake applied.
Errr.... what do you think is going to happen!? The automatic transmission is engaged and is going to fight against your handbrake.

Why would you ever (intentionally) put the car in drive with the handbrake applied?


Actually, to answer my own question, if you want to disable your daytime running lights while your car is running, you can apply the parking brake slightly. This is particularly useful at the drive-in so as not to have hundreds of people honking at you!
#42
Old 07-24-2013, 09:21 AM
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Quote:
Originally Posted by AaronX View Post
(Automatic) when in drive, the car would still move forward even with the handbrake applied.
Quote:
Originally Posted by Sparky812 View Post
Errr.... what do you think is going to happen!? The automatic transmission is engaged and is going to fight against your handbrake.
There are plenty of cars with effective parking brakes that will not move when in gear even if revved up a fair amount. A parking brake that won't hold against an automatic idling in gear isn't going to hold against some hills, and is either a poor design or in poor condition.
#43
Old 07-24-2013, 01:25 PM
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Originally Posted by Gary T View Post
There are plenty of cars with effective parking brakes that will not move when in gear even if revved up a fair amount. A parking brake that won't hold against an automatic idling in gear isn't going to hold against some hills, and is either a poor design or in poor condition.
I agree. A well-designed, well maintained parking brake should hold at least at an idle, but my point is that by engaging the transmission with the parking brake applied usually causes the damage which results in the "creep" the OP describes.

I'm just surprised that the OP doesn't realize the two opposing forces.
#44
Old 07-24-2013, 02:26 PM
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Originally Posted by Sparky812 View Post
Actually, to answer my own question, if you want to disable your daytime running lights while your car is running, you can apply the parking brake slightly. This is particularly useful at the drive-in so as not to have hundreds of people honking at you!
Alas, that disappeared between 2003 and 2008 with two of my GM cars.

Last edited by Kenm; 07-24-2013 at 02:27 PM.
#45
Old 07-25-2013, 01:22 AM
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Of course I know there are 2 forces. I expect a handbrake to be stronger, that's all. You're not supposed to use the handbrake when in gear?
#46
Old 07-25-2013, 08:00 AM
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Originally Posted by Kenm View Post
Alas, that disappeared between 2003 and 2008 with two of my GM cars.
That sucks, I know it works on my 2003 GMC truck, it worked on my old 1998 Grand Prix not sure about my wife's 2010 Sienna though. I'll try it, when I get a chance.
I'm in Canada, had(has) different regulations regarding daytime running lights than the USA.

Oddly coincidental to the dwindling numbers of drive-in theatres.

Quote:
Originally Posted by AaronX View Post
Of course I know there are 2 forces. I expect a handbrake to be stronger, that's all. You're not supposed to use the handbrake when in gear?
Not usually during regular driving, brakestands, doughnuts, 180s brake turns, etc. notwithstanding.

Last edited by Sparky812; 07-25-2013 at 08:04 AM.
#47
Old 07-25-2013, 01:21 PM
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Originally Posted by Sparky812 View Post
I'm in Canada. . . .
Me too.

I could keep the headlights off on the '03 and '92 with only two clicks of the parking-brake pawl, but not so with the '08.
#48
Old 07-25-2013, 09:51 PM
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I only recently started driving, and I see cars can move even with the hand brakes applied! There is a light in the dashboard that glows, and beeps that alert you, but to me this is completely counter-intuitive. Shouldn't cars be completely immobile with the hand-brakes applied?

Is this a problem with the car itself, or is there some logic to having things work this way?
#49
Old 07-26-2013, 12:40 AM
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Location: KCMO
Posts: 11,118
Quote:
Originally Posted by hniyer View Post
I only recently started driving, and I see cars can move even with the hand brakes applied! There is a light in the dashboard that glows, and beeps that alert you, but to me this is completely counter-intuitive. Shouldn't cars be completely immobile with the hand-brakes applied?
I wouldn't say completely immobile, but the parking brake should hold against rolling down a hill, and thus against mild acceleration when in gear. The warning light is meant to be a reminder that the parking brake is set so that the driver will release it before starting to drive.

Quote:
Is this a problem with the car itself, or is there some logic to having things work this way?
It's a problem.
#50
Old 07-26-2013, 11:41 AM
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Join Date: Feb 2007
Location: Minnesota
Posts: 1,086
Quote:
Originally Posted by hniyer View Post
I only recently started driving, and I see cars can move even with the hand brakes applied! There is a light in the dashboard that glows, and beeps that alert you, but to me this is completely counter-intuitive. Shouldn't cars be completely immobile with the hand-brakes applied?

Is this a problem with the car itself, or is there some logic to having things work this way?
Some vehicles have automatic adjusters for the parking brakes and some have to be adjusted by hand.
When brake shoes are replaced there are things that need to be addressed with parking brakes that aren't always done by the back yard miskanic.
And if the parking brake hasn't been a regular part of you driving and one day you decide to engage it, well the brake sometimes will not release fully and that is a big problem! You will be able to smell the hot brake linings but of course that can happen also after a period of hard braking, except the rear wheels brakes will be the hot ones with faulty P-brake release.
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