| > 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. |
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.