Just curious. After I replaced brake fluid on my 2010 MDX, I noticed the brake pedal was hard until I started the car and then it slowly depressed several inches. Since I don't drive the car, I am not sure what normal is although I believe my 99 Avalon does the same thing. Since brake fluid is incompressible, how can the booster vacuum "force" the pedal down? Thanks.
Why does brake pedal depress when engine runs?
- Thread starter artbuc
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When force is applied at the master cylinder the hydraulic pressure increases. In response, the pads of each pair move a bit closer together squeezing the the rotor and allowing some fluid into the calipers. The flexible rubber brake hoses also expand ever so slightly taking up a small amount of fluid. The pedal drops to compensate for the volume of brake fluid moved. With very little force the pads move less and the brake hoses don't expand at all. The booster uses vacuum (some other vehicles don't use vacuum) to multiply the force you apply to the brake master cylinder so you get enough brake system hydraulic pressure to move things.
With a non-functioning booster (which is what you have when the vacuum has been depleted) you'd probably need both feet to apply enough force to generate maximum stopping power. If you did this regularly the seat might just rip out of the floor pan. A master cylinder for a brake system without a booster would typically use a smaller bore to generate the pressure with less force but would need a longer stroke to displace as much fluid an thus there would be greater pedal travel required for braking.
Your MDX doesn't have a robot installed to swap in a small bore long stroke master when you shut off the engine and swap the normal master back in when you start the engine so with the engine off and vacuum depleted you don't have enough force available until you start the engine. This is the hard pedal with short stroke feel you experience while not moving. If you were at highway speed it might feel more like one warm wet leg that gets cold pretty quickly when you notice that the whole slowing down and stopping thing doesn't work very well.
With a non-functioning booster (which is what you have when the vacuum has been depleted) you'd probably need both feet to apply enough force to generate maximum stopping power. If you did this regularly the seat might just rip out of the floor pan. A master cylinder for a brake system without a booster would typically use a smaller bore to generate the pressure with less force but would need a longer stroke to displace as much fluid an thus there would be greater pedal travel required for braking.
Your MDX doesn't have a robot installed to swap in a small bore long stroke master when you shut off the engine and swap the normal master back in when you start the engine so with the engine off and vacuum depleted you don't have enough force available until you start the engine. This is the hard pedal with short stroke feel you experience while not moving. If you were at highway speed it might feel more like one warm wet leg that gets cold pretty quickly when you notice that the whole slowing down and stopping thing doesn't work very well.
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Thanks yonyon. I forgot to think about the system. Brake fluid is incompressible, but the brake system isn't. However, it is hard to imagine the system having enough give to account for the extra travel between booster off/ booster on. Seems like the brake hoses would be ballooning out. I guess you have to know the rod bore to calculate just how much brake fluid is involved. Maybe a whole lot less than I am imagining. Also, I have to look at where the rod connects to the pedal. Maybe several inches of pedal travel translates to a much smaller rod travel.
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I have had ABS systems that allow the pedal to slowly sink to the floor if you stand on them with about 50 lbs. In particular a 1997 ish GMC 3/4 ton diesel truck, that likely uses a non-vacuum booster.
I'm not real happy with that design, if in fact that's how it's supposed to work. Remember hearing about mid 90s GMC jimmys that were getting into wrecks: The ABS made the pedal sink and the drivers had the seats too far back so they ran out of leg and foot and weren't getting the pedal down enough. (Surely sounds like GMs explanation when I put it to type.
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I'm not real happy with that design, if in fact that's how it's supposed to work. Remember hearing about mid 90s GMC jimmys that were getting into wrecks: The ABS made the pedal sink and the drivers had the seats too far back so they ran out of leg and foot and weren't getting the pedal down enough. (Surely sounds like GMs explanation when I put it to type.
Originally Posted By: artbuc
Thanks yonyon. I forgot to think about the system. Brake fluid is incompressible, but the brake system isn't. However, it is hard to imagine the system having enough give to account for the extra travel between booster off/ booster on. Seems like the brake hoses would be ballooning out. I guess you have to know the rod bore to calculate just how much brake fluid is involved. Maybe a whole lot less than I am imagining. Also, I have to look at where the rod connects to the pedal. Maybe several inches of pedal travel translates to a much smaller rod travel.
Much less rod travel, yes.
Thanks yonyon. I forgot to think about the system. Brake fluid is incompressible, but the brake system isn't. However, it is hard to imagine the system having enough give to account for the extra travel between booster off/ booster on. Seems like the brake hoses would be ballooning out. I guess you have to know the rod bore to calculate just how much brake fluid is involved. Maybe a whole lot less than I am imagining. Also, I have to look at where the rod connects to the pedal. Maybe several inches of pedal travel translates to a much smaller rod travel.
Much less rod travel, yes.
eljefino, It's pretty common with a hydraulic booster that you can always press the pedal to the floor regardless of whether or not it's got ABS. If the driver had the seat too far back to press the pedal to the floor then either the driver wasn't qualified to operate that machine or the driver was careless.
There are some ABS systems using a vacuum booster that drop the pedal very low maybe an inch or so during an ABS stop but again, if you can't press the brake pedal an inch past what would normally be a hard stop.....
There are some ABS systems using a vacuum booster that drop the pedal very low maybe an inch or so during an ABS stop but again, if you can't press the brake pedal an inch past what would normally be a hard stop.....
Originally Posted By: eljefino
I have had ABS systems that allow the pedal to slowly sink to the floor if you stand on them with about 50 lbs. In particular a 1997 ish GMC 3/4 ton diesel truck, that likely uses a non-vacuum booster.
I'm not real happy with that design, if in fact that's how it's supposed to work. Remember hearing about mid 90s GMC jimmys that were getting into wrecks: The ABS made the pedal sink and the drivers had the seats too far back so they ran out of leg and foot and weren't getting the pedal down enough. (Surely sounds like GMs explanation when I put it to type.
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I had a 92 s-10 that worked like that, i unhooked the abs and the brakes then worked great!
I have had ABS systems that allow the pedal to slowly sink to the floor if you stand on them with about 50 lbs. In particular a 1997 ish GMC 3/4 ton diesel truck, that likely uses a non-vacuum booster.
I'm not real happy with that design, if in fact that's how it's supposed to work. Remember hearing about mid 90s GMC jimmys that were getting into wrecks: The ABS made the pedal sink and the drivers had the seats too far back so they ran out of leg and foot and weren't getting the pedal down enough. (Surely sounds like GMs explanation when I put it to type.
I had a 92 s-10 that worked like that, i unhooked the abs and the brakes then worked great!
Yonyon is absolutely right. Try this for grins. Park on a downhill. Make sure there is no traffic. Make sure your parking brake is up to the task. Turn the key to acc/ign position so that steering will not be locked. Pump the brake pedal few times to deplete all the vacuum.
Put the vehicle in neutral. Let the parking brake go. As your vehicle starts rolling down, use service brake to (try to) stop the vehicle.
Oh, you might also want to carry a change of underwear for this experiment
- Vikas
Put the vehicle in neutral. Let the parking brake go. As your vehicle starts rolling down, use service brake to (try to) stop the vehicle.
Oh, you might also want to carry a change of underwear for this experiment
- Vikas
Originally Posted By: Vikas
Oh, you might also want to carry a change of underwear for this experiment- Vikas
My wife would say "don't bother, you couldn't tell the difference".
Oh, you might also want to carry a change of underwear for this experiment
- VikasMy wife would say "don't bother, you couldn't tell the difference".
art -
What you describe is normal.
I can't believe you never noticed this on another car.
With vacuum in the booster, or vacuum from the engine running, the pedal will go down more.
With no vacuum [pump the brakes a few times with the engine off] , the pedal should be high and hard.
What you describe is normal.
I can't believe you never noticed this on another car.
With vacuum in the booster, or vacuum from the engine running, the pedal will go down more.
With no vacuum [pump the brakes a few times with the engine off] , the pedal should be high and hard.
Originally Posted By: mechtech2
art -
What you describe is normal.
I can't believe you never noticed this on another car.
With vacuum in the booster, or vacuum from the engine running, the pedal will go down more.
With no vacuum [pump the brakes a few times with the engine off] , the pedal should be high and hard.
After I calmed down I did remember my car behaves this way. But, I wasn't 100% sure if the pedal was hard BEFORE I started the car (getting old is a PITA!). Plus, it seemed like it dropped much more than "normal" with booster assist. I was just freaking out thinking I had sucked the MC down too low and would have to bench bleed it. I did that once to my Avalon. The brake line flare nuts on the MDX MC were torqued by an 800 lb gorilla. My car is down getting VC gaskets replaced and I am out of Honda BF. I know [censored] well that any quality DOT 3 BF is ok but I don't want to get into any warranty hassles with Acura.
art -
What you describe is normal.
I can't believe you never noticed this on another car.
With vacuum in the booster, or vacuum from the engine running, the pedal will go down more.
With no vacuum [pump the brakes a few times with the engine off] , the pedal should be high and hard.
After I calmed down I did remember my car behaves this way. But, I wasn't 100% sure if the pedal was hard BEFORE I started the car (getting old is a PITA!). Plus, it seemed like it dropped much more than "normal" with booster assist. I was just freaking out thinking I had sucked the MC down too low and would have to bench bleed it. I did that once to my Avalon. The brake line flare nuts on the MDX MC were torqued by an 800 lb gorilla. My car is down getting VC gaskets replaced and I am out of Honda BF. I know [censored] well that any quality DOT 3 BF is ok but I don't want to get into any warranty hassles with Acura.
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Cars vary, of course.
Some feel horrid , but brake just fine.
Some feel horrid , but brake just fine.
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