brake pedal locks up if you pump it several times with the car off

[BlahBlahYouToo]

why does this happen?
so if your car is rolling downhill (while off) and you hit the brakes, the car won't stop?
are the calipers already clamped to the rotors?

 

[funboy6942]

Loss of vacuum to the brake booster means you will have to put down some big time pressure on the pedal to get the car to stop. Meaning once all the air is gone you get a stiff/locked brake pedal at which point your parking brake becomes your friend in a emergency situation with the car turned off.

 

[BlahBlahYouToo]

well, the parking brake really doesn't stop the car.

so you lose all braking ability once the pedal locks up? so you really only have a few pumps before you're SOL in case of emergency, e.g. car stalls and you're moving at high speeds.

 

[BZeto]

Originally posted by: BlahBlahYouToo
well, the parking brake really doesn't stop the car.

so you lose all braking ability once the pedal locks up? so you really only have a few pumps before you're SOL in case of emergency, e.g. car stalls and you're moving at high speeds.

I'm pretty sure in that case (just the engine stalling) you should still have brake power. I know what you're saying about pumping the brake pedal and losing brake power with the car off, but once you turn the key on (without starting the engine) you should have brake power.

 

[Zenmervolt]

Originally posted by: BZeto

Originally posted by: BlahBlahYouToo
well, the parking brake really doesn't stop the car.

so you lose all braking ability once the pedal locks up? so you really only have a few pumps before you're SOL in case of emergency, e.g. car stalls and you're moving at high speeds.

I'm pretty sure in that case (just the engine stalling) you should still have brake power. I know what you're saying about pumping the brake pedal and losing brake power with the car off, but once you turn the key on (without starting the engine) you should have brake power.

Wrong.

Brakes on almost all cars use power assist that is provided by engine vacuum. If the engine is not running, there is only enough residual vacuum for 2-3 pumps of the pedal. Turning the key to "on" without starting the engine will not provide vacuum. (Very rarely there are brake systems that use hydraulic assist from the power steering system, these also do not provide assistance if the engine is not running, and diesel engines have a vacuum pump because a diesel engine does not generate vacuum on its own, but again, that doesn't work without the engine running.)

However, just because there is no vacuum assist does NOT mean there is no braking. The brakes will function 100% fine, just without power assist. This is the same way that your steering will still function without the engine running, just without assist. The brakes will require more pressure to stop, but it should not be terrible and they will still provide vastly more stopping power than just the emergency brake.

Prior to the 1980s, most cars did not have power brakes and most brake pedals felt "locked up" all the time. My old 914 does just fine without power brakes.

ZV

 

[alpineranger]

All cars have a check valve between the engine intake manifold and the brake booster. What this means is that you will not immediately lose brake booster vacuum if the engine stalls. However, once you've used up that vacuum it's not going to be replaced, of course. Practically speaking you'll have maybe 2-3 applications of the brake before you lose the vacuum assist completely. If you have a problem in the system however (bad check valve, vacuum hose leak past the check valve, bad brake booster), all bets are off.

 

[jagec]

Originally posted by: funboy42
Meaning once all the air is gone you get a stiff/locked brake pedal at which point your parking brake becomes your friend in a emergency situation with the car turned off.

Just jam on the pedal, you'll stop just fine. The parking brake sucks and is MUCH less effective than non-power assist regular brakes. Stand on that pedal and you'll be amazed by how fast it stops, even without the assist.

Kids these days, with their brake boosters and power steering and automatic transmissions...

 

[Shawn]

You can still stop without power assist.

 

[alfa147x]

Originally posted by: BlahBlahYouToo
well, the parking brake really doesn't stop the car.

so you lose all braking ability once the pedal locks up? so you really only have a few pumps before you're SOL in case of emergency, e.g. car stalls and you're moving at high speeds.

then open your door and use your foot

 

[LTC8K6]

Yeah, it just seems like you don't have any brakes, but you do. You're just used to it.

Sorta like when you lose the "power" part of your power steering. At first, it seems like the wheel won't turn, but it will, it's just harder to do.