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0 pointsbobmcnamara7mo ago0 comments

> The brakes on all Toyotas are fully hydraulic

No, most are hydraulic with vacuum boost.

If you aren't expecting it, the increased force required to drive the brakes hydraulically from the pedal without boost assist is significant and can be surprising. I assume most folks haven't had an engine fail going downhill, but for a large pickup I was standing on the brake pedal and had to push my leg down by pulling on the steering wheel to stop it.

> That seems.... hard to believe

Anyhow, the vacuum booster is driven from the engine airflow. At wide open throttle the vacuum available to the booster is minimal because the restriction is as open as it can be.

You can test how it feels by rolling at a medium speed in a parking lot, shifting into neutral, and killing the engine. The vacuum reservoir may provide you one or two brake pumps and then you're on your own.

Another test: after shutting down your car after a regular drive, try depressing the brake to the floor a few times. You'll soon exhaust the boost reservoir and the brake pedal will become very stiff now that it's fallen back to full hydraulic operation. In this condition if you hold the pedal halfway down when you start the car you'll feel the brake boost kick in soon as the engine starts.

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mnw21cam6mo ago

Note that in many engines, vacuum is provided by a separate vacuum pump, which is driven by the rotation of the engine or rarely by an electric motor. This is always true for a diesel engine (because there is no intake manifold vacuum) and many petrol engines too (if they are direct injection, they also may not have sufficient intake manifold vacuum).

Even if you have a total engine failure, if you're going down a hill, if you keep it in gear then the motion will be turning the engine still, which will be generating vacuum regardless of whether that is by a separate vacuum pump or the intake manifold (unless the throttle valve also fails or you get a gaping hole in the side of your intake, but that seems unlikely).

So braking should still keep working as normal, as long as you keep it in gear and don't disengage the clutch.

bobmcnamaraOP6mo ago

TIL! Haven't worked with anything that new.

Note that Toyota only introduced vacuum pumps in 2017, well after the changeover to electronic throttles. All plotted unintended acceleration complaints would've been without a separate vacuum pump.

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