Stopping a runaway Toyota: what's the problem?

[Rick]

@Magiver

The parking brake is a mechanical extension of whatever braking system the car has. It has more mechanical hp than the engine and will lock up the wheels unless it is a drum system that is out of adjustment.

Ummm, not always. I know of rear disc systems that have a little tiny drum brake inside the rear discs for a parking brake. Or a car where the parking brake is a separate little tiny brake caliper that mechanically applies a little tiny baby brake pad.

Now on to power brakes. These work by using the mechanical energy of a vacuum chamber. The chamber is drawn down through the vacuum created by the engine under normal load. If the engine is racing then no vacuum is available. Loss of power assist is NOT a loss of brakes. What occurs is a SUBSTANTIAL change in the amount of foot pressure needed to run the brakes. Instead of 5 lbs of pressure from your foot it may take 150 lbs of pressure. Most people are capable of generating this kind of pressure. It is important that a person understand that a loss of power assist does not mean a loss of brakes, it just means a much greater amount of pedal force is needed. Please note this is a different situation than a loss of braking due to a ruptured line or bad master cylinder which means you need to pump the brakes to recover whatever mechanical action is available.

Again, not quite. Close, but no cigar. There are one way valves in the vacuum system that prevents the loss of vacuum when the throttle is opened wide, so at WOT there still is vacuum in the brake booster for a stop or two. Also many modern cars are equipped with a separate vacuum pump to provide vacuum to the brake system when the engine is not developing an adequate amount of vacuum (used during cold start on modern engines with variable cam timing).
so the fact that the engine is at Wide open throttle does not equal no vacuum boost.

[Magiver]

@Rick

Ummm, not always. I know of rear disc systems that have a little tiny drum brake inside the rear discs for a parking brake. Or a car where the parking brake is a separate little tiny brake caliper that mechanically applies a little tiny baby brake pad.

OK, note to self, never use an absolute in a sentence. I bow to your automotive knowledge.

@Rick

Again, not quite. Close, but no cigar. There are one way valves in the vacuum system that prevents the loss of vacuum when the throttle is opened wide, so at WOT there still is vacuum in the brake booster for a stop or two. Also many modern cars are equipped with a separate vacuum pump to provide vacuum to the brake system when the engine is not developing an adequate amount of vacuum (used during cold start on modern engines with variable cam timing). so the fact that the engine is at Wide open throttle does not equal no vacuum boost.

While this is true I’ve seen the one way valve fail many times which was my assumption in the case listed. For whatever reason, the car didn’t have any power braking left and I wanted to point out that this does not mean the brakes won’t work for anyone driving a Toyota death machine.