Scary Incident Last Night In The A8
07-20-2004, 01:34 PM
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Scary Incident Last Night In The A8
I was in a friend's relatively steep driveway in DC, waiting for my passenger to come out. I had the radio on and headlights on. It took a while so when I was ready to go I forgot I hadn't actually started the car and that just the accessories were on. So I put the car in reverse and it just started SLIDING down the driveway into the street. The brakes did NOTHING! I put the car into park real quick and it stopped right away. Luckily I wasn't sliding very fast and there was no traffic!
Is this normal?
Is this normal?
07-20-2004, 01:50 PM
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Yes..
The brakes do not have any pressure when the engine is off. Your pedal is usually hard as a rock and will not do much. You were quick to think to put in neutral and start the engine.. otherwise grab the e-brake. Glad no one was hurt..
07-20-2004, 02:46 PM
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Without the car running, you do have brakes ...
but no vacuum assist; and it's a big assist. <b>Very</b> firm pressure is required to stop the car, but you can stop the car even from high speed if necessary, such as when the motor dies when you run out of gas. It sounds like you weren't expecting the big increase in pedal pressure, and it might not be a bad idea to try it again (but somewhere where you can safely roll to a stop).
The emergency brake will also work, but it's best left to use when you're stopping in a straight line. Because it operates only the rear brakes, using the emergency while cornering could initiate a spin.
Finally, you're lucky you didn't damage the transmission. "Park" usually just uses a pin to lock the transmission; engaging it while moving can shear the pin.
The emergency brake will also work, but it's best left to use when you're stopping in a straight line. Because it operates only the rear brakes, using the emergency while cornering could initiate a spin.
Finally, you're lucky you didn't damage the transmission. "Park" usually just uses a pin to lock the transmission; engaging it while moving can shear the pin.
07-20-2004, 03:33 PM
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While were on the subject...
How do we test if the vacuum assist is working or how do we tell when the master cylinder is bad..
On my A6 and the A8 i notice that regular braking is fine however the pedal gets very hard when initially hard braking.. once the initial hard brake is done the rest following are much better.
On my A6 and the A8 i notice that regular braking is fine however the pedal gets very hard when initially hard braking.. once the initial hard brake is done the rest following are much better.
07-20-2004, 04:31 PM
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Re: how to tell if i did shear or damage the pin?
As to testing it out, you can start the car in neutral without a problem?
I was rolling pretty slowly, but it was pretty scary that there were no brakes!
I was rolling pretty slowly, but it was pretty scary that there were no brakes!
07-20-2004, 05:20 PM
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did the same thing once with my dads 03 ls430... didnt realize the car was off
until i was out of the driveway and the brakes were firm. that was scary. never could tell if the engine was on or not, bc it was so quiet
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07-20-2004, 05:46 PM
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If the car stopped moving, the pin is intact, ...
but I wouldn't push my luck and test it this way again. Also, I just walked out and checked -- my '01 S8 does start when in neutral.
07-20-2004, 06:04 PM
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Re: How do we test
With the car not running, pump the brake pedal quickly a few times (it'll be very firm), and hold your foot lightly on the pedal when you start the car. If the pedal falls a significant amount, the vacuum assist is working ... it's done it's job by adding to your pedal effort.
The master cylinder is just like a piston pump, and the piston seals are designed to expand under pressure from the brake pedal. The more pressure on the brake pedal, the better the seal seals. The master cylinder can fail in several ways, but the most common symptom is that when stopped with your foot lightly on the brake pedal (at a stoplight for example), suddenly the pedal just goes to the floor. Usually it's initially intermittent, but it gets progressively worse. Hard pedal pressure expands the seals, and the brakes will work ... for a while.
The master cylinder is just like a piston pump, and the piston seals are designed to expand under pressure from the brake pedal. The more pressure on the brake pedal, the better the seal seals. The master cylinder can fail in several ways, but the most common symptom is that when stopped with your foot lightly on the brake pedal (at a stoplight for example), suddenly the pedal just goes to the floor. Usually it's initially intermittent, but it gets progressively worse. Hard pedal pressure expands the seals, and the brakes will work ... for a while.
