flat said:

The rears wouldn’t lock up normally, under heavy braking there is a valve that deliberately stops that happening

It’s called the G valve, and is on the drivers side (RHD) chassis dumb iron, below the master cylinder. Follow the lines and you will find it. Cant be adjusted and doesn’t normally fail.

when you emergency stop you won’t lock up all 4 wheels, by having less brake effect on the rear it keeps the vehicle straight under heavy braking

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kevstar said:

Never heard of a G valve heard of G spot tho .....load sensing valve or brake bias valve.

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Al2O3 said:

I thought the weight of the engine and box apply more downward force on the front axle than there is on the rear. This creates far greater friction between the tyres and the road making it much harder to stop the front wheels turning. The rear wheels have less force pressing down on them and so would lock up if the same braking force was applied. So a bias valve is used to proportionately split the required force between front and back. If you had an old standard car and pulled the hand brake on the rear would not overtake the free running front unless you slowed the front by turning the steering, this would also create a turning force about the front wheels causing the car to pivot about that point and the rear end would start to rotate past the front.

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flat said:

on a perfect surface only surely? Perfectly flat in all directions with, perfectly centralised steering , perfectly matched tyres and balanced brakes etc but in reality camber, undulations, slightly different tyre pressures etc, one brake fractionally stronger than another, mass slightly off centre (driver in seat etc.) and all could make you spin if you yank that handbrake

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